Xii PHYLUM CHORDATA 493 



the opening of the vulva. The tail is very short and covered 

 with a tuft of fluffy hair. 



The fore- and hind-limbs, both of which take part in loco- 

 motion and in supporting the weight of the animal, differ 

 considerably in size the fore-limbs being much shorter 

 than the hind-limbs. Both have the same general divisions 

 as in the lizard. The upper arm is almost completely 

 hidden by the skin, being applied closely against the side of 

 the body. The manus is provided with five digits, each 

 terminating in a horny claw. The thigh is also almost hidden 

 by the skin ; the pes has four digits only, all provided with 

 claws. 



The spinal column of the rabbit is divisible, like that of 

 the pigeon, into five regions the cervical, the thoracic, the 

 lumbar, the sacral, and the caudal. In the cervical region 

 there are seven vertebrae ; in the thoracic twelve or some- 

 times thirteen, in the lumbar seven, or sometimes six, in the 

 sacral four, and in the caudal about fifteen. 



The centra of the vertebrae in a young rabbit consists of 

 three parts a middle part, which is the thickest, and two 

 thin discs of bone the epiphyses anterior and posterior, 

 applied respectively to the anterior and posterior faces of 

 the middle part or centrum proper. Between successive 

 centra in an unmacerated skeleton and thin disc-like plates 

 of fibro-cartilage are the inter-vertebral discs. 



The first vertebra or atlas (Fig. 298, B) resembles the cor- 

 responding vertebra of the pigeon in being of the shape of 

 a ring without any solid centrum like that of the rest. On 

 the anterior face of its lateral portions are two concave arti- 

 cular surfaces (art) for the two condyles of the skull. The 

 second vertebra or axis (B and C) bears on the anterior face 

 of its centrum a peg-like process the odontoid process (od) 

 which fits into the ventral part of the ring of the atlas. 



