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In this article we will discuss about the axial and appendicular skeleton of toad.
1. The Axial Skeleton:
The axial skeleton forms the axis of the body;
It consists of:
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(a) The skull and
(b) The vertebral column. and
(a) Skull:
The broad and flat skull supports the head.
It is composed of the following parts:
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(i) The brain case or tranium for enclosing the brain;
(ii) The nasal or olfactory capsules containing the organs of smell;
(iii) The orbits for housing the eyes;
(iv) The orbit or auditory capsules for lodging the ears;
(v) The upper jaw, which is fused with the ventral margin of the skull;
(vi) The lower jaw, which is movably articulated to the upper jaw; and
(vii) The hyoid apparatus, supporting the floor of the mouth cavity.
The cranium forms the narrow central part of the skull. It is tubular in shape and pierced by a large hole at the posterior end. This is the foramen magnum (foramen=bore) through which the brain joins the spinal cord. The foramen is bordered on each side by a cartilage bone called ex-occipital, which bears a convex occipital condyle.
The two occipital condyles fit into depressions on the first vertebra. The roof of the cranium is formed by a pair of thin flat bones called front parietals which extend from the ex-occipitals behind to the nasal capsules in front.
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The floor of the cranium is supported by a dagger-shaped Para sphenoid, the blade of which points forwards. At the anterior end of the cranium is a ring-like cartilage bone, the sphenethmoid, which is completely covered over by the front parietals. Besides the foramen magnum, the brain case is perforated by several small openings for the exit of the cranial nerves.
The nasal capsules are attached to the anterior end of the cranium and the sphenethmoid extends into them from behind. Each half of the nose is covered over by a triangular membrane bone called nasal, and its floor is supported by another membrane bone, the vomer.
The orbits are represented by two large cavities, one on each side of the cranium. They are meant for lodging the eyes. The cartilaginous optic capsules are not fused with the cranium and therefore they are not found in a prepared skeleton.
The auditory or otic capsules are situated posteriorly, one on each side of the cranium, just in front of the ex-occipitals. Each capsule is a cartilaginous box which is covered over by a single cartilage bone called pro-otic.
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The floor of the capsule is supported by a lateral expansion of the Para sphenoid. On the outer side of the pro-otic is a thin hammer-shaped membrane bone, the squamosal, which connects the otic capsule with the posterior end of the upper jaw.
The upper jaw forms the outer margin of the skull. It consists of two halves which are united to the cranium, both in front and behind. Each half of the jaw is composed of an outer series of three membrane bones, the premaxilla, the maxilla and the quadratojugal. The premaxillae are small bones at the anterior part which unite with one another in the middle line and are fused to the nasal capsules.
Behind the premaxilla is a long slender maxilla which is joined to the quadratojugal. At the posterior end of the quadratojugal is a small cartilaginous piece, the quadrate, which bears a concave facet for articulating with the lower jaw.
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Extending between the cranium and the outer series of three bones just described, there is an inner series of three supporting bones. These are the palatine, the pterygoid and the quadrate. They serve to brace up the upper jaw with the cranium.
The palatine is a rod-like bone in front of the orbit which connects the maxilla with the sphenethmoid. The pterygoid is a Y-shaped bone, the foot of which is attached to the junction between the palatine and the maxilla.
The two arms of the pterygoid are directed backwards; the inner arm supports the floor of the otic capsule and the outer arm projects towards and fuses with the quadrate. Thus the pterygoid and the quadrate form an efficient girder for the articulation of the lower jaw.
The lower jaw consists of two halves which are united anteriorly, in the middle fine, by a short ligament. Each half is derived from a Meckel’s cartilage and is composed of three bones, the mentomeckelian, the angulosplenial and the dentary.
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The mentomeckel in is a small cartilage bone at the extreme anterior end. Behind the mento-meckelian is a long slender angulosplenial which encloses the inner margin of the Meckel’s cartilage. The posterior end of the angulosplenial articulates with the quadrate of the upper jaw.
The dentary is a membrane bone which covers the outer surface of the angulosplenial but does not extend up to the posterior end. Both jaws are toothless.
The hyoid apparatus is a cartilaginous plate lying on the floor of the mouth cavity. It form$ the surface for the attachment of the tongue. The main bulk of the apparatus is known as the body of the hyoid.
A pair of horns called anterior cornua extend from the anterior part of the body to the auditory capsules. Similarly, a pair of posterior cornua project backwards from the body of the byoid and enclose the laryngotracheal chamber or voice box. The anterior cornua are much longer than the posterior cornua.
(b) Vertebral Column:
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The vertebral column is situated dorsally and extends from the base of the skull to the posterior end of the trunk. It is composed of a series of ten separate bones, the first nine of which are the ring like vertebrae and the last is a long rod-like urostyle (oura = tail; styles rod).
A typical vertebra, taken from the middle of the series, consists of a solid cylindrical part, the body or centrum, which is concave in front and convex behind and therefore said to be procelous (pro = front; coel=hollow). Dorsally, the centrum bears a bony ring, the neural arch, which encloses the spinal cord in a neural canal. On the roof of the neural arch is a low median ridge, the neural spine or spinous process.
On each side, the neural arch carries an outwardly directed transverse process. Extending forward from the anterior margin of the neural arch are two spoon like articulating processes called pre-zygapophyses; and similarly, two posterior articular processes or post-zygapophyses project backward from the posterior margin of the neural arch.
When the vertebrae lie in their natural position, the convex posterior surface of one centrum fits into the concave anterior surface of the following centrum, and the post-zygapophyses of the vertebra in front overlap the pre-zygapophyses of the vertebra behind. The vertebrae cannot separate, because they are held together by ligaments.
A flexible vertebral column is produced in this manner. It encloses the spinal cord in a continuous neural canal formed by the ring-like neural arches of the successive vertebrae. There are intervertebral foramina between consecutive neural arches, through which spinal nerves come out.
The first vertebra or atlas articulates with the skull. It has neither transverse processes nor pre-zygapophyses. Instead, it carries on its anterior face two concave facets for articulating with the occipital condyles of the cranium. Its centrum is greatly reduced and flattened, and its neural arch bears a pair of post-zygapophyses for articulating with the pre-zygapophyses of the second vertebra.
The following seven vertebrae resemble the typical vertebra.
The ninth or sacral vertebra is the largest of the series. Its centrum is procoelous; but the posterior end of the centrum bears two rounded knobs, the condylar facets, which fit into corresponding concavities on the urostyle. Its transverse processes are stout and fan-shaped; sloping outwards, they articulate with the pelvic girdle. Since the ninth vertebra is the last, it has no post-zygapophyses.
The urostyle represents a number of fused vertebrae. It is a long slender rod which extends from the ninth or sacral vertebra to the posterior end of the trunk. It bears a median neural crest dorsally, and a pair of socket-like concavities at its anterior end for articulating with the centrum of the ninth vertebra.
2. The Appendicular Skeleton:
The appendicular skeleton is composed of:
(a) The pectoral or shoulder girdle,
(b) The pelvic or hip girdle,
(c) The forelimbs, and
(d) The hind limbs.
(a) Pectoral Girdle:
The pectoral girdle is a bony framework encircling the anterior part of the trunk. It consists of two symmetrical halves which are united with one another in the mid-ventral line, but separated dorsally.
Each half of the pectoral girdle is composed of the following structures. Lying dorsally and above the vertebral column is a broad, partly cartilaginous plate called supra-scapula. Attached to the supra-scapula is a stout scapula or shoulder blade, which forms the outer side of the girdle.
Connected with the lower end of the scapula and lying ventral to the vertebra column are two rod-like bones, a clavicle in front and a coracoid behind. The clavicle is a thin membrane bone which encloses the precoracoid cartilage. The precoracoid being en-sheathed is not easily visible.
The inner ends of the clavicle and the coracoid are joined by a curved piece of cartilage, called epicoracoid. The right epicoracoid is slightly overlapped by the left one. The clavicle, the coracoid and the epicoracoid are separated from one another by a wide gap which is known as the coracoid fontanelle.
There is a cup-shaped glenoid cavity at the junction of the scapula, the clavicle and the coracoid. The head of the arm bone fits like a ball into the socket of the glenoid cavity.
The two halves of the pectoral girdle are united by the epi-coracoids in the mid-ventral line, and the breast-bone or sternum projects backward from the united posterior ends of the two epi-coracoids. The xiphisternum is a flattened cartilaginous plate forming the terminal part of the sternum.
It is to be noted that no part of the pectoral girdle is directly articulated to the vertebral column. The girdle is kept in normal position by strong muscles, and if these are cut the whole framework may be slipped along the long axis of the body.
The pectoral girdle performs the following functions:
(1) Besides supporting the forelimbs which can move freely in the glenoid cavities, the girdle encloses and therefore protects important organs, such as heart and lungs;
(2) It serves for the attachment of muscles which are concerned with respiration.
(b) Pelvic Girdle:
The pelvic girdle is a V-shaped structure at the posterior end of the trunk. The apex of the V is directed backwards and the two free ends of the V form a complete hoop by articulating anteriorly with the transverse processes of the ninth vertebra. The posterior end of the pelvic girdle is disc-like, with a cavity called acetabulum on either side. The acetabulum supports the hind limb by articulating with the thigh bone.
The disc itself is formed by the fusion of three bones on each side, the ilium, the ischium and the pubis. The two ilia form the anterior and dorsal sectors of the disc; they extend anteriorly as slender rods, one on each side of the urostyle, and articulate with the ninth vertebra.
The two ischia are completely fused to form the posterior sector of the disc. The ventral sector of the disc is formed by a triangular cartilage which represents the two pubes.
There is a general resemblance between the pectoral and the pelvic girdles, the scapula corresponding with the ilium, the coracoid with the ischium, and the precoracoid surrounded by the clavicle with the pubis. At the junction of the three elements of the pectoral girdle there is a cup-shaped glenoid cavity.
A very important point of difference is the direct articulation of the pelvic girdle with the vertebral column. The ilia are attached to the fan-shaped transverse processes of the ninth or sacral vertebra. Such direct articulation with the vertebral column is not found in the pectoral girdle.
(c) Forelimbs:
Each forelimb is composed of several bones. The arm is supported by a long bone called humerus, the ends of which are swollen and the middle part or shaft is slightly curved. Its rounded proximal end, the head, fits into the cup-shaped glenoid cavity to form the shoulder joint.
Its distal end carries a pulley-like trochlea for articulating with the forearm bone. Extending from the head to about the middle of the shaft is a prominent crest called deltoid ridge which serves for the attachment of muscles.
The forearm is supported by a single bone called radio-ulna which is formed by the fusion of two bones, radius and ulna. The proximal end of the radio-ulna is concave and articulates with the trochlea of the humerus to form the elbow-joint.
It is drawn out into a backwardly directed projection, called olecranon process, which extends beyond the trochlea and lies behind the elbow. The distal end of the radio- ulna is expanded and flattened for joining with the wrist.
The wrist or carpus is composed of six small pieces of carpal bones which are arranged in two rows, a proximal row and a distal row, each consisting of three carpals. The palm of the hand is supported by four slender rod-like bones, the metacarpals.
There are four fingers or digits, each of which is composed of bones called phalanges (singular =phalanx). The innermost finger corresponding to our thumb is absent. The first and second fingers have two phalanges in each, but the third and fourth have three in each.
(d) Hind Limbs:
Each hind limb is a system of bones. The thigh is supported by a long bone called femur, with a slightly curved shaft and swollen ends. Its rounded proximal end or head articulates with the acetabulum forming a ball and socket hip joint, and its distal end is slightly expanded to form a condyle for articulating with the shank bone.
The shank or crus is supported by a single bone called tibiofibula which is formed by the union of two bones, tibia and fibula.
The tibiofibula is expanded at both the ends; its proximal end articulating with the femur forms the knee joint, and the distal end articulating with the tarsals forms the ankle joint. In tarsal bones.
The two proximal tarsals are the calcaneum and the astragalus. Both are elongated and united with each other at their ends, but widely separated in the middle. The distal tarsals consist of two or three small pieces, which owing to their smallness are easily overlooked.
The foot or metatarsus is composed of five slender metatarsals. There are Jive toes or digits. The first and second toes have two phalanges in each, the third has three, the fourth four, and the fifth three. The great toe or hallux has a small projection on its inner side. This is known as the calcar or pre-hallux, and is composed of one or two small nodules.
The two pairs of limbs are built on the same plan. Both are essentially pentadactyle, that is ‘five-fingered’; but the thumb or pollex is absent in the forelimbs. A comparison between the forelimb and the hind limb reveals that the humerus corresponds to the femur, the radio-ulna to the tibiofibula, the carpals to the tarsals, the metacarpals to the metatarsals, and the phalanges of the fingers to those of the toes.
The bones of the hind limb are of course longer than the corresponding bones of the forelimb and the proximal tarsals are very much longer in comparison to the proximal carpals.
Joints:
In the construction of the skeletal framework bones are joined with one another in various ways. The term joint is applied to the place of union between bones. The joints are planned in such a manner that the animal derives the best mechanical advantage out of them.
The bones of the cranium need not move at all. They therefore are joined by interlocking sutures which permit no movement. The bones of the girdles are also locked by sutures. In the limbs, the bones are to move freely. In a movable joint the articulating surfaces of the bones are covered by smooth articular cartilages.
The joint as a whole is enclosed by a loose bag of connective tissue which is known as the articular capsule. The capsule is strengthened by bands of fibrous tissue called ligaments, and lined internally by a smooth, delicate membrane, the synovial membrane. The joint-cavity, between the two bones, contains a colourless synovial fluid which serves for lubrication.
There are two main types of movable joints:
(1) Ball and socket joint and
(2) Hinge joint.
The shoulder and the hip are ball and socket joints which permit movement of bones in every plane. The elbow and the knee are hinge joints permitting movement in one plane only. A hinge joint does not allow side-to-side movement.
