THE SKELETON OF THE FROG

The skeleton consists of the following parts :—

Vertebral column and skull.

Pectoral girdle and pelvic girdle.

Fore and hind limbs.

The Vertebral Column

The vertebral column is short and consists of only nine vertebra?, the last of which is followed by a bony rod called the urostyle. The first vertebra or atlas has a small centrum and has no transverse processes or anterior zygapophyses. It articulates with the skull by small hollow surfaces on the front of the vertebra?, which fit over a pair of small knobs at the back of the skull. The axis is firmly attached by centrum and zygapophyses to the vertebral column. The second to the eighth vertebra? are similar, and agree in general structure with a typical vertebra as shown by the rabbit. Each consists of centrum, transverse processes, anterior and posterior zygapophyses, and neural arch with a small neural spine. The centrum of each has a concave anterior surface and a convex posterior surface. The eighth vertebra differs from the previous vertebra? in having both surfaces of the centrum concave. The centrum of the ninth or sacral vertebra is convex in front, and behind has two small rounded knobs which fit into hollows on the front end of the urostyle. The transverse processes of the ninth vertebra are very large and are directed obliquely backward to join the hip girdle.

The Skull

The skull is made up of:—

A narrow box or brain case containing the brain.

Two olfactory capsules attached to the front end of the brain case, enclosing the organs of smell. Two auditory capsules attached to the hind end of the brain, containing the organs of hearing. The upper and lower jaws.

The upper jaw bears a row of small conical teeth and is immovably fixed to the olfactory and auditory capsule. On each side of the skull, between the brain case and the upper jaw, is a large space, the orbit or eye socket, in which the eye is lodged. The lower jaw has no teeth and articulates with a pair of projections directed downwards and backwards from the auditory capsules.

The Pectoral Girdle

The pectoral girdle has the form of an inverted arch, consisting of two halves joined in the mid-ventral line.

The upper part of each half is made up of a broad bone, the supra-scapula, which is attached at its ventral end to a narrow flat bone, the scapula. The lower part of each half is formed by two parallel bones, the clavicle and coracoid. The clavicle is narrower than the coracoid and lies in front of it. On each side where the coracoid and clavicle meet the scapula is the glenoid cavity for articulation with the head of the humerus. A bony rod tipped with cartilage projects in front of the girdle from the junction of the two clavicles, and a similar rod projects behind from the junction of the two coracoids. These rods form the sternum of the frog.

The Fore Limb

Both limbs of the frog are built on the principle of the pentadactyl limb, but show slight modifications of the primitive type. In the fore limb the radius and the ulna have become joined to form one bone, the radio-ulna, and only four complete digits are represented, the thumb being vestigial. Only six carpals are present.

The Pelvic Girdle

The pelvic girdle is V-shaped. The arms of the V project forward and are the ilia, and at their free ends they are attached to the transverse processes of the sacral vertebra. Where the ilia join each other there is a disc of bone with a hollow, the acetabulum, on each side, into which the head of the femur fits. The disc of bone is formed partly by the fused bases of the ilia, and partly by fused ischia and pubes.

The Hind Limb

The hind leg is very long and appears to have one joint more than the typical pentadactyl limb. This is because two of the four tarsal bones are much elongated and provide the leg with an extra segment. The tibia and fibula are fused together to form a single bone, the tibio-fibula, and the five digits are long and slender. A small claw-like structure occurs on the inner side of the big toe.

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