The first, second, and fifth digits have two phalanges apiece, the third and fourth have three.
Figure 31 B, showing a Newt's tarsus copied from Gegenbaur, has precisely the arrangement generally regarded as primitive for the higher vertebrates, except that tarsalia 4 and 5 are fused.
[54] i.e. between one vertebra and the next.
THE SKELETON OF THE FROG[55] (_Rana temporaria_).
The skin of the frog is smooth and quite devoid of scales or other exoskeletal structures. The only exoskeletal structures met with in the frog are:--
1. The =teeth=, which are most conveniently described with the endoskeleton.
2. The horny covering of the calcar or prehallux (see p. 167).
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.
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=.