II. ENDOSKELETON.
The endoskeleton of the adult frog consists partly of cartilage, partly of bone and each of these types of tissue occurs in two forms. The cartilage may be hyaline, as in the omosternum and xiphisternum, or may be more or less calcified as in part of the suprascapula and the epiphyses of the limb bones. The bone may be cartilage bone, or membrane bone.
The skeleton is divisible into an axial portion consisting of the skull, vertebral column, and sternum, and an appendicular portion consisting of the skeleton of the limbs and their girdles.
1. The Axial Skeleton.
A. The Vertebral column.
The vertebral column is a tube, formed of a series of ten bones which surround and protect the spinal cord. Of these ten bones nine are vertebrae, while the tenth is a straight rod, the urostyle, and is almost as long as all the vertebrae put together. The second to eighth vertebrae inclusive have a very similar structure, but the first and ninth differ from the others.
Any one of the second to eighth vertebrae forms a bony ring with a somewhat thickened floor, the centrum or body, which articulates with the centra of the immediately preceding and succeeding vertebrae. The articulating surfaces are covered with cartilage and are procoelous, or convex in front and concave behind. The eighth vertebra is however amphicoelous or biconcave. The centrum of each vertebra encloses an isolated vestige of the notochord. The neural arch forms the roof and sides of the neural canal, which is very spacious in the anterior vertebrae, but becomes more depressed in the posterior ones. The arch bears the neural spine, a low median ridge of variable character, and is drawn out in front and behind, forming the two pairs of articulating surfaces or zygapophyses by means of which the vertebrae are attached together. Of these the anterior articulating surfaces or prezygapophyses look upwards and slightly inwards, while the posterior articulating surfaces or postzygapophyses look downwards and slightly outwards. The sides of the neural arches are drawn out into a pair of prominent transverse processes. Those of the second vertebra look somewhat forwards, those of the third look directly outwards or somewhat forwards, while those of the fourth, fifth, and sixth are directed slightly backwards, and those of the seventh and eighth nearly straight outwards. All the transverse processes are terminated by very small cartilaginous ribs.
Special vertebrae.
The first vertebra is a ring-like structure with a much depressed centrum. It bears in front two oval concave surfaces for articulation with the condyles of the skull, while the centrum is terminated behind by a prominent convex surface. There are as a rule no transverse processes, and the postzygapophyses look downwards and outwards. Occasionally however transverse processes do occur. Projecting forwards from the centrum is a minute process better developed in the Newt. This resembles an odontoid process, and it has hence been supposed that the first vertebra is homologous with the axis of mammalia, and that the atlas of the frog is fused with the skull.
The ninth vertebra has very stout transverse processes directed backwards and somewhat upwards. They articulate with the pelvic girdle and hence this vertebra is regarded as the sacrum. The neural arch is much depressed, the centrum is convex in front and bears on its posterior surface two short rounded processes for articulation with the urostyle.