Mammals: Endo-Skeletal, Digestive, Respiratory and Bother Systems in Mammals

The following points highlight the top seven types of system in mammals. The types are: 1. Endo-Skeletal System 2. Digestive System 3. Respiratory System 4. Blood Vascular System 5. Nervous System 6. Excretory System 7. Reproductive System.

Type # 1. Endo-Skeletal System:

Skull:

Variation in the structure of skull is present amongst the different members of the class. But all the members share certain fea­tures in common. In the general plan of the laying of skull bones, the shadow of a reptilian ancestry is clearly indicated.

The most noteworthy feature of the mam­malian skull is the increased capacity of the cranium. The cranium is expanded dorsally and laterally to house the much-developed brain. The other feature of mammalian skull is that, excepting the mandibles, hyoid and audi­tory ossicle, all component bones are firmly united and their line of union forms sutures.

The articulation of the upper jaw and mandible is at the glenoid or mandibular fossa of the squamous region of the temporal rather than at the quadrate. This type of articulation is known as craniostylic or amphicraniostylic type of jaw suspension.

A reduction in the number of skull bones has appeared in mammals. The reduction is due mainly to loss of certain bones and fusion of others. The number of bones in the skull is about 35. Bones of the ancestors that have dis­appeared are pre-frontals, post-frontals and post-orbitals. Bones that have become reduced are the quadrate of the upper jaw, articular of the lower jaw and the hyomandibular.

The quadrate has formed the incus, the articular has formed the malleus and hyomandibular has formed the stapes. Incus, malleus and stapes constitute the ear ossicles. Cases of fusion are encountered in the temporal region. Another common area of fusion is found in the cartilage bones of the sphenoid complex.

The basal portion of the frontal often unites with the orbitosphenoid to form the presphenoid bone. The basisphenoid and alisphenoid may also fuse. Pleurosphenoid bone is absent in mammals. Epipterygoids derived from palatopterygoquadrate bar occupy its place. These are then known as alisphenoid.

The four separate bones of the occipital region are fused in some mammals forming a single occipital bone. Otic bones are fused to form the petrosal enclosing the internal ear. The petrosal may fuse with squamosal and tym­panic forming the petrosus region of the tem­poral bone.

Mammals Lack a Supra-Temporal Arcade:

But the infra-temporal arcade or zygomatic arch is present. As a result, mammals possess a single temporal fossa. An anteriorly projec­ting process from the squamosal and a posteri­orly projecting process from the jugal unite to form the zygomatic arch. The orbit is formed by lacrimal, maxillary, orbitosphenoid, palatines and sometimes ethmoid.

The orbital and temporal fossae are often confluent but, in many cases, they remain separated by a bar formed by a downwardly projecting postorbital process of the frontal joining a dorsal pro­jection of the jugal. The nasal cavities of mammals are large.

The nasal cavities are surrounded by pre-maxillaries, maxillaries and nasals. Mesethmoid is a vertical cartilaginous plate separating the right and left nasal cavities. A cribriform plate separates the nasal cavity from the cranial cavity.

A hard and bony secondary palate is found in mammals. Pre-maxillary, maxillary and palatine bones participate to form it. In whales and edentates pterygoid forms a part of the palate. Pterygoid is reduced in other mam­mals.

The lower jaw or mandible articulates with the squamosal. It is made up of a single piece, called dentary. The articulating surface of the mandible is the condyle. A coronoid process from the posterior portion of the mandible extends into the temporal fossa. An angular process at the posterior ventral angle of the mandible is prominent in many mam­mals. The two halves are united in front by a symphysis.

A typical mammalian hyoid apparatus consists of a body or basihyal and two pairs of cornua or horns. The anterior cornua are called styloid processes and are attached to the otic region of the skull.

In some forms a chain of bones between otic region and basi­hyal are found. These bones are known, respectively, as tympanohyal, stylohyal, epihyal and ceratohyal. The posterior cornu is often small and called thyrohyal which is con­nected to the thyroid cartilage of the larynx.

Vertebral Column:

The vertebrae are amphiplatian or acoeolus. The centra are formed from pleurocentra and the hypo-centra form intervertebral disc. The centra of mam­malian vertebrae—excepting Monotremes and Sirenia—bear epiphysis at either end during early phase of development. But, in adult forms, these become fused with the centrum.

The mammalian vertebral column is divided into five distinct regions. They are cer­vical, thoracic, lumbar, sacral and caudal. The numbers of cervical vertebrae is 7 and are a constant feature in most mammals. The length of neck is determined by the length of individ­ual vertebra rather than by an increase in num­ber.

However, the number of cervical verte­brae is 6 in two toed sloth Choloepus, 9 in three toed sloth Bradypus. In the sirenian manatee, Trichechus there are 6 cervical verte­brae. Fusion of some or all cervical vertebrae is seen in whales, marsupial mole, jerboas, etc.

The first cervical vertebra is called atlas. The atlas is provided with a pair of concavities for articulation with the occipital condyles of the skull. The second cervical vertebra is called axis.

It is provided with a peg-like odon­toid process. Cervical ribs are absent excep­ting Monotremata. The presence of foramen transversarium or vertebraterial canal for the passage of artery, vein and a sympathetic nerve plexus in the cervical vertebrae is very characteristic of mammals. The seventh and sometimes the sixth vertebrae may lack the foramina. In Perissodactyla the cervical verte­brae are opisthocoelous.

The number of thoracic vertebrae is vari­able. The minimum number are 9 as in Hyperodon. The greatest number are 25 as in sloths. Fusion of thoracic vertebrae in living mammals is not known. Thoracic ribs are pre­sent in the thoracic vertebrae. These ribs con­nect the vertebrae directly or indirectly with the sternum. Articular facets that are found in the thoracic vertebrae are tubercular facets and costal demifacets.

The lumbar vertebrae are large and strong­ly built. Usually there are 4 to 7 lumbar verte­brae. In the whale, Neobalaena, the number are 2 while in dolphin the number are 21-24. The transverse processes of the lumbar verte­brae are directed forward.

Some of the anteri­or lumbar vertebrae are provided with Mammillary processes or Metapophysis. The metapophyses of the armadillos are large and blunt and help to support the carapace.

The sacrum consists of several vertebrae firmly ankylosed together and serves for artic­ulation of pelvic girdle. The number of sepa­rate elements that fuse are variable. There are five such elements in man and horse, 3 in dog and cat and 4 in pig.

Extra elements coming from lumbar or caudal regions are often incor­porated. The vertebrae are pierced by sacral foramina both on the dorsal and ventral sides. The first sacral vertebra is provided with late­ral masses which bear cartilage-covered auri­cular surfaces into which the ilium fits in.

The numbers of caudal vertebrae are extremely variable. In man there are 3 or 4 caudal vertebrae fused to form the coccyx and in Manis there are 50 caudal vertebrae. The anterior caudal vertebrae bear neural arch, spine, zygapophysis, etc.

But these structures gradually diminish in size towards the tail end and thus the caudal vertebrae at the posterior end are represented by centra alone. Chevron bones are found in whales and edentates.

Sternum:

The sternum of mammals is composed of a series of separate bones arranged in linear order. Three distinct regions are recognised in the sternum. They are ante­riorly placed presternum or manubrium, a middle mesosternum consisting of several sternebrae and a posterior metasternum or xiphisternum from which hangs the xiphoid cartilage.

The presternum is keeled in bats. The sternal elements are fused in Cetacea, Sirenea and Primates. In man the entire struc­ture is flat and is made up of three parts—an anterior manubrium, middle body and posteri­or xiphoid process.

The clavicle and first pair of ribs join the manubrium. The second pair of ribs join the sternum at the point of junctions between manubrium and body. Five more pairs of ribs join the body of the sternum in an even manner, No rib reaches the xiphoid pro­cess.

Ribs:

Mammalian ribs are made up of two distinct regions—an upper and bony vertebral region and a lower cartilaginous region. The lower region joins the sternum directly or indi­rectly and is called costal cartilage. The ribs of monotremes are with three regions. Uncinate process is absent in mammalian ribs. Mammalian ribs are usually bicipital.

The part of the rib between the two heads is called capitulum and tuberculum. The remainder is called shaft. The area where the shaft is very much curved is called the angle of the shaft.

The ribs which are directly in contact with the sternum are called true ribs. False ribs are the posteriorly located ribs. The costal carti­lages of such ribs are either united with the costal cartilage of the last true rib or float freely. False ribs which terminate freely are called floating ribs.

The thoracic ribs are well-developed in mammals. The tubercular head of the rib articulates with the tubercular facet located on the ventral side of the transverse process and the capitulum articulates with the demi facet located in between two adjacent vertebrae. The number of mammalian ribs is vari­able. There are 9 pairs of ribs in whales and 25 pairs in sloth. Number of true ribs ranges from 3-10 pairs.

Girdles and Limb Bones:

Pectoral girdle:

In monotremes each half of the pectoral girdle consists of scapula, coracoid and precoracoid. The coracoids establish a connection with the manubrium of the sternum on the ventral side. Pre-coracoids join with the episternum which lies anterior to the sternum. The coracoids take major share in the formation of the glenoid fossa. Clavicles and interclavicles are present.

In higher forms the precoracoid and interclavi­cles are lost. The coracoid becomes reduced to a coracoid process which lies on the scapu­la adjacent to glenoid fossa. Scapula is pro­vided with scapular spines in most cases and the spine ends in acromian process. In some mammals the clavicle persists as a strong arch. In others it is lost. Presence of strong clavicle is correlated with the freedom of movement of the forelimb.

Forelimb bones:

The humerus bears a supracondylar foramen near its distal end in most mammals. The brachial artery and medi­an nerve of the arm pass through the foramen. Brachial vein does not pass through it. Radius and ulna are connected with the distal end of the humerus by a hinge joint which allows movement in one plane.

In primates the radius and ulna are not arranged in fixed positions, but are articulated in such a way that the dis­tal end of the radius can rotate about the ulna. The ulna articulates with the humerus through the olecranon fossa of the humerus by a notch. The proximal end of the ulna is called olecra­non process or elbow.

Carpal bones are many with a tendency towards fusion. Metacarpals are long. Number of phalanges in the first digit is 2 while in the rest the number are 3. In horse, only the 3rd or middle digit persists. This digit bears hoof. 2nd and 4th digit may remain as splints. In cattle the 3rd and 4th digits are functional. Each con­sists of three phalanges. The terminal phalanx bears the splitted hoof.

There are two metacarpals in cattle. Of these, one is larger and the other is small. The large one is formed by the fusion of third and fourth digits and the line of fusion can be seen. Small metacarpal is a vestige of the 5th digit.

In bats, the forelimbs are modified. All the elements of the forelimb are elongated but the ulna is very much reduced. The proximal end of ulna is present and its distal end is fused with the radius.

The first digit is free and bears a claw. Metacarpals and remaining digits are much elongated. The third digit is the longest. The pentadactyle limbs of cetaceans and sirenians are webbed to form the flippers. The central digit is with more than usual number of phalanges.

Pelvic girdle:

Ilium, ischium and pubis are the constituent bones of the pelvic girdle. The elements unite to form a single bone—inno­minate. Both ischia and pubes form symphysis in many mammals but in others the ischia do not meet ventrally so that only pubic symph­ysis is present. The three bony elements in most cases are united at the acetabulum.

A small acetabular (cotyloid) bone is frequently present in the acetabulum as a substitute for pubis or in some cases for ilium. A large obtu­rator foramen bounded by pubis and ischium is characteristic of mammals. The ilium and sacrum are firmly ankylosed.

The ilium in pri­mates and specially in man is broad and flat in correlation with upright posture. Pelvic girdle and hind limbs are absent in whales and Sirenia, but remnants of paired pubes and ischia are present. In monotremes and marsupials, from each pubis a marsupial bone or epipubic bone ex­tends towards the ventral wall of the abdomen.

Hindlimb bones:

Femur is strongly built. The head of the femur articulates with the Acetabulum. Trochanters on the femur are usually three in number. A patella or knee cap is always present. The tibia are stronger than the fibula. The fibula is reduced in size or is fused with the tibia forming a small splint. Cnemial crest on the fibula is present.

Tarsal bones are distinct entities. The proximal tarsal bones are represented by astragalus and calcaneum. The shape of the astragalus varies in dif­ferent mammalian orders. The astragalus bears at its proximal end a grooved surface called trochlea which articulates with tibia and fibu­la. The neck and head of astragalus are of varying shape.

In lower mammals an astragalar foramen is present at the outer border of trochlea. The heel of the foot is formed by a projection from the calcaneum. Reduction of the number of digits is more pronounced in the hind limb than that in the forelimb.

Mammals exhibit three types of foot pos­ture. The most primitive is the plantigrade pos­ture exhibited by man, bear and some insectivores. In plantigrade animals the whole foot remains in contact with the ground during locomotion. Digitigrade posture is exhibited by cat, dog, etc.

In this posture only the digits remain in contact with the ground and the wrist and ankle remain elevated. The unguligrade posture is most specialised and is exhibited by swiftly moving animals like horse, deer, etc. In this posture only the hoof remains in contact with the ground.

Type # 2. Digestive System:

The basic structure of the digestive system in mammals is more or less constant. Some of the parts associated with the system depict modifications. The modifications are dis­cussed below.

Tongue:

A muscular tongue is present on the floor of the buccal cavity of all mammals. The tongue shows many modifications in form but not in function. Typically, a mammalian tongue is lined by stratified squamous epi­thelium which often forms lingual papillae.

Papillae are of three types:

(a) Filiform most numerous, without taste bud,

(b) Fungiform resembles a mushroom, with taste bud, and

(c) Circumvallate largest papilla, lodged with­in a trench. Loose areolar connective tissue forms the core of the papillae.

Taste buds con­tain two types of cells—gustatory and sub-tentacular. Bundles of stratified muscle fibres inter­lace in all directions (Fig. 10.126). The tongue is movable in all groups of mammals excepting the whales.

The tongue muscles of ant-eaters extend as far as the sternum. The tongue acts as prehensile organ in most ruminants and anteaters. In the cat family, the filiform papillae are highly cornified and are used in rasping the flesh off the bones. In dogs, the tongue helps in the maintenance of body temperature because the sweat glands on the skin play little role in temperature regulation.

Type # 3. Respiratory System:

The basic respiratory organs are same as in guinea-pig. The lungs are actually formed by round, transparent and irregular air cells or alveoli (Fig. 10.143). Each alveolus is lined by large irregular and flattened squamous epi­thelial cells. Desquamated dust cells may be seen. Bronchioles and lymphoid tissue may be present.

The lungs show modifications in different mammals. In Cetacea, the epiglottis and arytenoid are long forming a tube which extends into the nasal chamber. A third bronchus is present in some Cetacea and Artiodactyla which opens into the right lung in front of the ordinary bronchus. Air cavities are often associated with various parts of the res­piratory tract.

An eustachian tube is present connecting the tympanic cavity to the pha­rynx. Air sinuses of considerable dimensions may sometimes remain present into the bones of skull—specially the maxilla and frontals. These spaces are called maxillary antra and frontal sinuses, respectively. Pharyngeal air- sacs are present in howler monkey—Alouatta.

Type # 4. Blood Vascular System:

The blood of mammals is warm and blood temperature ranges between 35°C to 40°C, excepting the monotremes which are cold­blooded. The mature RBC are non-nucleated and circular in outline excepting the camels in which the RBC are elliptical in outline.

In the venous system in most mammals, only the right precaval persists and the left precaval aborts and its vestige gives rise to coronary sinus. In monotremes, marsupials, elephants, rodentia, insectivora and chiroptera both right and left precavals persist. In monotremes all the pulmonary veins open by a common trunk.

In metatheria and eutheria there are four pulmonary veins. These veins either open into the heart separately or two veins of one side unite to form a single lateral trunk which opens into the heart. An abdomi­nal vein is present in Tachyglossus.

The presence of peripheral or sub-peripheral anastomoses is a very peculiar feature in many mammals. This is a means for clima­tic thermal adaptation and is present in whale, sirenia, seals, amphibious, rodents and sloths.

With the help of this arteriovenous retes these creatures can swiftly loose heat or ensure heat retention. Hibernation is practised by many small mammals, specially the bats and rodents. The North American ground squirrel (Citellus beecheyi) can be induced to hibernate irres­pective of season.

Type # 5. Nervous System:

Development of Central Nervous System:

The nervous system develops from the embryonic ectoderm. Along the mid-dorsal line of the whole length of the body above the notochord, the ectoderm forms a thickened neural or medullary plate. By the process of differential growth and migration of cells the flanges of the plate become raised up into folds, called neural folds (Fig. 10.148A-B).

The neural folds approach each other forming a neural groove in the midline. As develop­ment goes on, the edges of the groove fused along the midline and are converted into a neural tube. The neural tube eventually becomes separated from the overlapping epidermal ectoderm. The cavity enclosed in the tube is called the neurocoel that opens to the exterior through a pore, called neuropore.

The neuropore finally closes and development of neural tube completes. The neural crest cells (Fig. 10.148C-D), appearing in two longitudinal bands, one on either side segre­gate from the developing neural tube. The brain was primarily a tube-like structure (Fig. 10.149), which due to unequal growth, torsions and flexions transforms into a compli­cated adult structure.

Structural Differentiation of the Neural Tube:

The anterior part of the neural tube is trans­formed into the brain and the posterior part forms the spinal cord. These two parts are demarcated from one another by a constric­tion, called isthmus. During the organogenesis of the brain, remarkable changes occur at the anterior part, the encephalon or primitive brain.

The first step of differentiation of the brain is the development of three evaginated portions, called primary brain vesicles and finally these three vesicles are recognised as prosencephalon or fore-brain, mesencepha­lon or midbrain and rhombencephalon or hindbrain (Fig. 10.150).

The prosencephalon is again subdivided into telencephalon and diencephalon and the rhombencephalon into metencephalon and myelencephalon, while the mesencephalon remains undivided.

A deli­cate membrane that runs at the anterior wall of the telencephalon, called lamina terminalis (Fig. 10.151) and the roof of the cerebral hemi­spheres or cerebrum is called cortex or palli­um. The ventrolateral wall of the cerebrum becomes thick and called corpus striatum.

Mammalian central nervous system or brain is relatively large amongst the vertebrates. The structure of the cerebral hemispheres of the forebrain is very complex. The cerebral hemispheres are large and project forward above the olfactory lobes and posteriorly above the diencephalon and midbrain. A deep median fissure separates the two hemispheres dorsally.

Each hemisphere is again separated into an anterior frontal lobe and a postero­lateral temporal lobe by an oblique Sylvian fissure. The roof of the cerebral hemispheres is invested with neopallium formed by the up growth of the lateral parts of the pallium.

The neopallium is best developed with large number of neurons. The neurons form a layer, called cortex. Below the cortex lies a thick layer of meduliated nerve fibres forming the medulla. Thus neopallium is made up of outer cortex and inner medulla.

The cortex bears many depressions, called sulci and lobes, called gyri. These are formed by in-folding of the cortex to increase the surface area. The hemispheres are smooth in Orninthorhynchus, somewhat convoluted in Tacpyglossus. In lower marsupials no convolution is present.

The cerebral hemispheres of two sides are connected with each other along the middle line by a band of nerve tissue, called corpus callosum—a structure unknown in other verte­brates and it is even absent in monotremes and marsupials. The floor of the hemispheres is thick and forms anteriorly the corpora striata and posterolaterally the hippocampus.

The hippocampus is a derivative of the archicortex in the shape of an marching band encroaching upon the lateral ventricle. It is concerned with the memory of spatial relationship.

An ante­rior commissure joins the two corpora striata and the two hippocampi are joined by a hippocampal commissure. A delicate membrane, called lamina terminalis, runs between the anterior commissure and optic chiasma. The lateral ventricles within the hemispheres are complex.

Each ventricle consists of a middle body from which three prolongations or cornu run forward, backward and outward. The olfactory lobes are small and lie below and in front of the hemispheres.

Each olfactory lobe runs backward into an olfactory tract. Each tract terminates in a round elevation, called tuberculum olfactorium. The olfactory tracts are kept separated from the hemispheres by rhinal fissure. The rhinocoel or olfactory ventricle is ill-developed.

The diencephalon is small and is almost covered dorsally by the cerebral hemispheres. Internally the third ventricle or diocoel is laterally compressed. Its roof is delicate and vascular. It is called velum interpositum. It is continuous with the choroid plexus of lateral ventricles. Above the choroid plexus lies the ill-developed and stalked pineal body or epiphysis.

The side walls of the diencephalon or thalami are thick mass of white and grey matters. The two thalami are connected together by a habenular commissure which lies close to the pineal stalk. The floor of the diencephalon is called hypothalamus.

The hypothalamus bears on the ventral side the optic chiasma and the infundibulum. The peculiarity of the mammalian optic chiasma is that all the fibres of the optic nerve do not participate in forming it, but only the inner fibres decussate or cross. The pituitary gland is attached with the infundibular tip. Behind the infundibulum lies a round corpus albicans or corpus mamillare.

The midbrain bears on its dorsal side two pairs of optic lobes, called corporaquadrigemina, which is characteristic of mammals. The anterior pairs of optic lobes are connected with each other by a delicate posterior com­missure. The floor of the midbrain is thick forming the crura cerebri.

The cerebellum is large and complex and is divided into a number of lobes. There is a large central lobe, called vermis, two lateral lobes and two outer or floccular lobes. The cerebellar surface is convoluted by the formation of sulci and gyri.

On the ventral side a thick band of nerve fibre, called pons Varolii, joins the cere­bellar cortex of two sides. A large number of tracts of nerve fibres or peduncles join the cere­bellum to the other parts of the brain. The medulla oblongata is thick.

Table 60 represents a comparative account of brain of toad, lizard, pigeon and guinea-pig.

Human Brain:

Human brain is larger among vertebrates but not the largest, nor the largest in relation to body weight and nor even the most convolu­ted.

Location:

It is located within a bony case, cranium which protects the brain from external injuries.

Volume:

The volume of the human brain is 1380 cc.

Coverings (Meninges) and Cerebrospinal fluid:

The brain is covered by three meninges (coverings).The outer covering of the brain which is fibrous and tough, called Dura mater. Next to Dura mater, is a narrow space boun­ded by a fine, delicate covering, called arach­noid mater. Next to arachnoid mater, a narrow space, called subdural space, contains a small amount of tissue fluid.

The inner thin, vascular covering which remains close contact with brain, called pia mater. The space between the arachnoid layer and pia mater is called subarachnoid space that contains cerebro­spinal fluid. The subarachnoid space is not connected with the subdural space directly but is continuous with the ventricular system of the brain through three openings in the roof of the 4th ventricle.

Of the three openings, the lateral two openings are called foramina of Luschka and medial opening is called foramen of Magendie. The cerebrospinal fluid is a clear colourless fluid whose sp. gr. is 1005. The pH is about 7.33 and remains more or less con­stant. Its composition is given in Table 59.

Cavities of Brain:

The brain is not a compact mass. It bears fluid-filled intercommunicating cavities. The cavities within the brain are called ventricles and the fluid is called cerebrospinal fluid. The cavities in the cerebral hemispheres are called lateral ventricles or first and second ventricles. The diencephalon or thalamencephalon con­tains third ventricle or prosencoel.

The lateral ventricles communicate with the third ventricle by an aperture, called foramen of Monro. The fourth ventricle is situated in the medulla oblongata. The third ventricle is connected with the fourth ventricle by an aquous-filled narrow passage, called aqueduct of Sylvius or iter. The cavity within the optic lobes is called optocoel and that of the olfactory lobes rhinocoel.

Divisions of brain (Fig. 10.153A-B):

Human brain like other mammals is divi­sible into 3 regions on the basis of develop­ment, namely:

(i) Forebrain (Prosencephalon),

(ii) Mid-brain (Mesencephalon) and

(iii) Hind limb (Rhombencephalon).

Forebrain (Prosencephalon):

Forebrain is again subdivided into telence­phalon and diencephalon (or thalamencepha­lon). Prosencephalon is formed by the olfactory lobes and cerebrum (cerebral hemispheres).

Cerebrum:

Cerebrum is the largest part of the brain. It is divided into two equal parts hemispheres by a median longitudinal groove. The two hemis­pheres are called jointly cerebrum. Each cere­bral hemisphere is again divided into 5 lobes by 4 fissures. These lobes are called frontal lobe, parietal lobe, temporal lobe, occipital lobe and limbic lobe.

The surface of cerebral cortex is marked by furrows, called sulci and ridges of convolutions or regions between sulci, called gyri. The lateral and central sulcus are the main two sulci. The frontal lobe of the cerebral hemi­sphere is situated in front of central sulcus and parietal lobe is behind the central sulcus.

The area between the lateral sulcus, called the late­ral lobe. The most posterior part of the cerebral hemisphere is the occipital lobe. The limbic lobe is sometimes applied to the region which lies on the medial side of the hemisphere adja­cent to the corpus collosum.

Cerebral cortex (Fig.10.153B):

The outer layer of cerebral hemispheres which is composed of grey matter, varies 1.25 mm to 4 mm thick, called cerebral cortex or neopallium or pallium. It is thinner in the occipital region than the frontal.

Cerebral medulla:

The two cerebral hemispheres are con­nected ventrally by a transverse band of nerve fibres, called corpus callosum.

Functions of Cerebrum:

The cerebrum is the seat of memory, intel­ligence, thinking, reasoning and learning. It controls will, emotions and speech. It interprets various stimuli or sensations. The tem­poral and occipital lobe is associated with specific senses, such as visual area, hearing areas, olfactory area or speech centre.

Olfactory lobes:

Olfactory lobes are less developed in human beings and much developed in other animals in which sense of smell is well-deve­loped. It is associated with smell. The lobes are incorporated with the cerebrum. They are seen from the ventral side.

Diencephalon:

The part of forebrain which lies between telencephalon and mesencephalon, called the diencephalon (thalamencephalon). The cavity within the diencephalon is called third ventri­cle (diocoel). The lateral walls of the caudal end develop two large masses of grey matter, called thalami. The floor of the third ventricle forms the hypothalamus. Thalami are concerned with emotions and recognise heat, cold and pain.

Mesencephalon:

It lies between the fore-brain and hind brain and consists of 4-lobes, called corpora quadrigemina. The roof of the midbrain is called tectum and the thickened ventral side is known as crura cerebrei or cerebral pedun­cle. The optic lobes are connected with vision, olfactory reflection and muscle con­traction.

Hind brain (Rhombencephalon) Metencephalon (Cerebellum):

It occupies ¹/₁₀ of the human brain and consists of a superficial layer of grey matter, called the cortex, overlying a layer of white matter. On the basis of embryological and phylogenetic the cerebellum is divided into 3 lobes.

They are anterior, middle and poste­rior lobes. The anterior and posterior lobes are more older than the middle, called palaeocortex. The middle lobe is much developed and called neocerebellum. The deeper central part of the cerebellum which is composed of white matter, called medulla. Cerebellar nuclei of grey matter are scattered in the white matter which contains fibre tracts that connect medulla and cerebrum.

The efferent fibres of the cerebellum are fewer than afferent fibres. The outgoing impulses originate in the Purkinge cells and are relayed by cerebellar nuclei. The dentate nucleus is the largest and is a crumpled contour. The cerebellum regulates and co­ordinates contraction of skeletal muscle. It controls posture, equilibrium and voluntary movements.

Myelencephalon (Medulla):

It includes medulla oblongata. The side walls of medulla oblongata are thick and formed of white matter. The roof—the medulla, is thin and contains a plexus of blood vessels, known as posterior choroid plexus. Most of the vital functions are regulated by this region.

Most of the cranial nerves develop from the lateral side of the medulla and these are centres of vital func­tions, such as respiratory, cardiac and gastric functions.

Type # 6. Excretory System:

The kidney in mammals is metanephric and is the principal excretory organ. There are a pair of kidneys. Each kidney is a compact, bean-shaped structure and is retroperitoneal in position. It is covered over by a connective tissue covering, called capsule. The median side of each kidney bears a depression, called hilum. At this point the ureter and renal veins leave the kidney and renal artery and nerve enter the kidney.

The kidney proper is demarked into two distinct regions—an outer cortex and an inner medulla. The renal corpuscles and convoluted portions of the tubules remain confined in the cortical region.

Each renal corpuscle is made up of a Bowman’s capsule formed by an undented expansion at the cortical end of the excretory tubule. Into the Bowman’s capsule lies a capillary bed formed by an artery. The capillary bed is called glomerulus (Fig. 10.155).

The wall of the glomerulus is one cell thick. Each tubule, as it leaves the Bowman’s capsule, becomes differentiated into four dis­tinct regions. They are a proximal convoluted tubule, a long loop of Henle with descending and ascending portions and a distal convolut­ed tubule.

Secretory tubules with their Bowman’s capsules are numerous. It is esti­mated that there are about one million renal corpuscles in each kidney. Each distal convo­luted tubule opens in a collecting tubule.

The medulla is made up of large area. The area is occupied by a number of renal pyra­mids. Columns of cells, called renal column of Bertini, originate from the cortex and invade the spaces in between the renal pyramids. The apex of the pyramids is subdivided into smaller units, called lobules.

The collecting tubules remain confined usually into the pyra­mids. The inner border of each pyramid is projected into an out-pocketing of the pelvis. This out-pocketing is called minor calyx. Several minor calyces join together and form major calyces which, in their turn, open into the pelvis.

The centre of the kidney is the sinus of the metanephros. It is filled up with blood vessels, nerves, fatty tissue, pelvis and calyces. The ureter emerging from the pelvis passes through the hilum and opens into the urinary bladder.

Fluid is removed from the blood by the renal corpuscles and is passed down the secre­tory and excretory tubules. Most of the filtrate, specially glucose and salts (about 99%), is reabsorbed through the walls of the tubule and is returned to the blood stream.

The remainder is called urine. Urine is made up of water, urea, sodium chloride, potassium and uric acid. This straw-yellow fluid passes down the ureters, remains for some time stored in the urinary bladder and then is voided through the urethra.

The urinary bladder is a muscular sac and is derived from the ventral wall of the cloaca and a portion of allantoic stalk. The ureters open directly on the dorsal posterior side of the bladder in all mammals excepting monotremata.

In many mammals (Proboscidea, Cetacea, Artiodactyla, etc.) the kidney retains embryo­nic lobulated condition. In others the kidney surface is smooth in adult stage. In Pro-cavia urine is voided as viscid steaks. The animals live in hot and rocky area and, as such, have developed a mechanism for the retention of as much fluid as possible and as a result the urine is of thick consistency.

Type # 7. Reproductive System:

Male Reproductive System:

This system consists of a pair of testes, ducts and associa­ted glands. The testes are oval bodies and remain in exceptional cases in their original site of formation in the abdominal cavity. Thus the testes are abdominal in cetacea, ele­phants, hyracoidea and in some insectivores.

In majority of the mammals they pass down the inguinal canal and lie in a space between the urinogenital and anal aperture. The space is known as perinaeum. The testes may descend down further and may remain inside a pendulus pouch of skin, called scrotum. The scrotal sac may sometimes be double.

Each testis is covered over by a mem­brane made up of connective tissue fibres. The membrane is called tunica albuginea. Internally the testes are composed of a large number of seminiferous tubules and Leydig’s or interstitial cells (Fig. 10.156). The Leydig’s cells are richly supplied with blood and are located in the interstices between adjacent seminiferous tubules.

These cells secrete the male sex hormone. The spermatozoa from each testis are discharged from the seminifer­ous tubules into a plexiform rete testis, from where they go to the epididymis. The epi­didymis continues as vas deferens which runs downwards. At the posterior end the vas deferens bear two swellings.

The first swelling is called ampulla and the second swelling is called seminal vesicle which is glandular. These vesicles do not store seminal fluid but, in all probabilities, produce fructose to be utilised by the spermatozoa. Ejaculatory duct coming out from the vesicle enters into the median urethra and penis.

A small diverticulum, called uterus masculines, is present at the proximal part of the urethra. The uterus masculines is in all proba­bilities, a rudimentary Mullerian duct. Surrounding this region lies a large and alveo­lar prostate gland. It is absent in monotremata, marsupials and cetacea.

Ducts from a pair of small and racemose Cowper’s glands or bulbourethral glands open into the urethra near the base of the penis. Both prostate and Cowper’s glands are under nervous and endocrine control and their secretions are added to the spermatozoa.

The structure of penis is variable in diffe­rent mammals but basically it is composed of sinusoidal and erectile tissue arranged in lon­gitudinal columns. Two such columns on the dorsal side make up the corpora cavernosa of penis.

These columns are attached proximally to the ischia excepting in monotremes, marsu­pials and some edentates. A third column of erectile tissue, called corpus spongiosum, occupies a ventral position. It is perforated by the urethra and distally it becomes dilated to form the glans penis.

An osseous element, called os penis or os priapi, is present on the septum between the columns of corpus caver­nosa in some rodents, carnivores, bats and some primates. This structure is absent in man and whales. According to some its homologue is the clitoris of the female.

At the time of sexual excitement nerve impulses cause dilatation of the arterioles and capillaries filling up the sinuses inside the penis. The plain muscle of each sinus wall is now relaxed and contraction of circular mus­cles occurs. Thus the blood flow is impeded causing turgidity and erection of penis. Most of the terrestrial carnivores bear horny spines or copulatory grapples near the tip of the penis.

Much experimental work has been done on the mammalian testis and it has now been confirmed that production of male hormone is done by the Leydig’s cells. The male hormone is called testosterone. Synthetic manufacture of testosterone is now possible and any substance having equal physiological properties similar to that of testosterone is called androgen.

Castration is a long practice for the improvement of the quality of flesh of domes­tic animals and to make them docile. A cas­trated bull is known as steer, a castrated horse is called a gelding, a castrated boar is called barrow and a castrated man is called eunuch.

If an animal is castrated before puberty, mas­culine structures and characteristics do not develop. Castration after puberty leads to regression of accessory sex organs but secon­dary sex characters are little altered. It has been shown that the internal secretion of testes has a profound role on the development of certain part of skeleton specially the pelvis.

Female Reproductive System:

It consists of a pair of ovaries and a pair of oviducts. The ovaries are compact bodies located in the lumbar or pelvic regions. During development they arise at the level of the metanephric kid­neys but later descend to their permanent and most posterior location. Mammalian ovaries are small and remain suspended from the mid-dorsal body wall by mesovarium.

Blood vessels and nerves enter the ovary at a point, called hilum. The hilus is attached to the uterus by a suspensory ligament. The ovaries are so oriented that the surface is exposed to the coelomic cavity but the funnel of the oviduct lies so close to the ovary that the dis­charged ovum directly enters into the funnel.

The whole or part of the ovary remains en-sheathed by a specialised peritoneal fold, called germinal epithelium. The ovary is made up of two distinct regions—the outer cortex and inner medulla. The medullary region is composed of connective tissue, blood and lymphatic vessels, smooth muscles and nerve fibres. The cortex consists chiefly of compact stroma.

The Graafian follicles lie in the cortex. The follicles vary in size according to their states maturity. Three different types of follicles according to maturity can be recognised.

They are primary, growing and mature follicles. Generally primary follicles lie near the ovarian surface, growing ones advance towards the medulla and the mature ones extend through­out the thickness of the cortex and may pro­duce elevations on the ovarian surface.

A primary follicle consists of a centrally located ovum surrounded by a single layer of epithelial follicular cells. In growing follicles the follicular cells become arranged in many layers. Each follicle now becomes surrounded in a sort of capsular arrangement. The cap is called theca (Fig. 10.157). The theca is arranged in two layers—the outer layer is called theca externa.

The cells of the theca externa are long and spindle-shaped. The inner layer of the theca is made up of round cells and is called theca interna. It is believed that the cells of the theca interna secrete female sex hormone or oestradiol. The ovum in a mature follicle ceases to enlarge and becomes surrounded by a clear, thick and transparent envelope.

The envelope is pro­duced by the follicular cells and is called zona pellucida. The envelope is pierced by minute canals through which nourishing materials enter the ovum. The follicular cells that lie just outside the zona pellucida are columnar in shape, stain differently and appear very con­spicuous. They form layer and are known as corona radiata.

The follicular cells which were solid mass of ceils so long now undergo vacuolation and rearrangement. This results in the formation of a cavity, called antrum or follicu­lar cavity. The cavity remains filled up with a semi viscous fluid, called liquor folliculi or fol­licular fluid.

The ovum surrounded by zona pellucida and corona radiata lies on a small hillock of follicular cells and projects into the antrum. The hillock is called germ hill or cumulus oophorus. The follicular cells at the periphery are several layer in thickness and is collectively known as stratum granulosum.

Many follicles start developing at a time in mammals but most of them degenerate and become atretic. In a mature female, at perio­dic intervals which are typical of each species, one or more follicles grow to maturity, the number being closely related to the number of offsprings produced at a time.

Fully developed ovum comes out by rupture of the follicle and comes to the Fallopian tube through the funnel of the oviduct. In cat, rabbit and shrew ovula­tion does not occur unless they copulate.

In some mammals the antrum of follicle after the discharge of the ovum becomes filled up with a clot of blood coming from the rup­tured blood vessels of the theca. Such a body is called corpus hemorrhagicum. In other mammals the cells of the stratum granulosum and theca interna undergo transformation.

They enlarge and push into the antrum obli­terating it. A large and round mass of cells, called corpus luteum, finally fills the antrum. Each corpus luteum persists as a scar-like corpus albicans. The corpus luteum acts as important endocrine gland. It secretes a hormone, called Progesterone, which pre­pares the reproductive tract for pregnancy and for maintenance of pregnancy.

Mullerian ducts or oviducts in paired con­dition are present in all mammals. In higher mammals the duct is differentiated into three distinct regions. The upper region is called Fallopian tube. It is narrow and its free end opens into the coelome forming a fimbriated funnel.

The middle part of the duct has become much enlarged to form the uterus. The uterus leads to a terminal vagina which receives the penis of the male during copulation. The lower part or neck of the uterus is called cervix.

Fusion of various degrees of the oviducts of two sides occurs in mammals. The most primitive condition is present in monotremes. Here the two oviducts remain separate and terminate separately in the urogenital sinus.

Each duct is differentiated into anterior narrow Fallopian tube and posterior expanded uterus. The uterus is beset with calcareous glands and the Fallopian tube with albumen glands. It has been seen that in platypus only the left oviduct is functional.

In marsupials oviducts are paired and the two vagina open into a urogenital sinus. In some the vaginae are fused at their anterior ends forming a vaginal sinus which extends posteriorly as a blind pocket. This is some­times called third vagina.

In eutherian mammals a single vagina formed by the fusion of the distal parts of the oviducts is present. The uterine portions of the oviducts show different degrees of fusion resulting different types of uteri. The most primitive type of uterus of eutherian mammals is called uterus duplex (Fig. 10.158A) in which the two uteri open separately into the vagina.

Uterus duplex is present in many rodents and elephants. The fusion of the two uteri at their lower ends results in the formation of uterus bipartitus (Fig. 10.158B). Such uterus opens to the vagina by a single aperture. Bipartitus uterus is found in carnivore, pig, cattle, a few bats and in some rodents.

As a result of still greater degree of fusion the formation of uterus bicornis occurs (Fig. 10.158C). Such uterus is found in sheep, whales, insectivores and most bats. Complete fusion of the two uteri has formed uterus simplex (Fig. 10.158D). Uterus simplex is present in man and apes.

The external part of the female reproduc­tive organ is called vulva. In primates two- folds of skin, the labia minora, are present at the margins of the vestibular opening. In some apes and human female two additional folds called labia majora are present at the margins of vulva. They correspond to the scrotal swellings of man.

The glands that are associated with the female genital system are:

(a) Glands of Bartholin,

(b) Para-urethral glands and

(c) Vestibular glands.

Glands of Bartholin are homologous to the Cowper’s gland of males. The gland secretes a clear, viscid fluid under sexual excitement and the fluid acts as lubri­cant during copulation.

Para-urethral glands are homologous to the prostate glands of males. But the presence of this gland is not constant in all mammals and its function is not known. Vestibular glands are located at the base of urethra and on clitoris. They are mucus-secreting glands.

Breeding season:

Most mammals breed more or less in specific times of the year. But in man, baboon, giraffe and elephants, no par­ticular mating season is present. Animals with a single cycle in each sexual season are called monoesters animals. Dog, koala, kangaroo are examples of monoesters mammals. Polyestrous mammals are those in which sexual season appears many times a year.

Sheep, rat, goat are examples of polyestrous mammals. Post-ovulatory destruction of endometrium results in menstruation. It is present in ‘catarrhines’, certain primates and African elephant and shrew. Fig. 10.159 gives an idea of the relationship between ovary and uterine wall with different harmfulness in man.

During men­struation, the uterine wall undergoes degener­ation and is sloughed off. Menstruation is asso­ciated with the preparation of uterus for the implantation of fertilized ovum.

The cycle occurs approximately every 28 days in female during the reproductive life. Menstruation is associated with a series of hormonal events. The ovaries secrete a hormone, called estro­gen. Estrogen is responsible for the develop­ment of secondary sex characters and acces­sory sex organs of the female. Progesterone is another hormone secreted by the corpora lutea.

Progesterone is concerned with the changes occurring in preparation for and dur­ing pregnancy. Relaxin is another hormone or hormonal agent. It has been extracted from the corpora lutea of cows, the blood stream of pregnant rabbits, dogs and cows. Whether it is a true ovarian hormone is questionable since the site of its formation is still obscure. Relaxin helps in bringing out formative changes for and during pregnancy.

Experimentally it has been shown that the removal of ovaries before puberty prevents further development of uterus, vagina and Fallopian tube, all of which remain in infantile condition. Extirpation after puberty brings about atrophy of organs. Removal of ovaries or corpus luteum alone during pregnancy brings about abortion.