XIII 



PHYLUM CHORDATA 449 



The sacral vertebrae are firmly ankylosed together to form a 

 single composite bone, the sacrum. The vertebrae bear a close 

 resemblance to those of the lumbar region, but the hypapophyses 

 and anapophyses are wanting, and the metapophyses are com- 

 paratively small. The first and second bear great expanded lateral 

 processes, or sacral ribs, with roughened external surfaces for 

 articulation with the ilia. These are the only sacral vertebra in 

 the strict sense of the term, the following two being in reality 

 anterior caudal. 



Of the caudal vertebras the more anterior resemble those of the 

 sacral region, and have similar processes ; but as we pass back- 

 wards in the caudal region all the processes gradually diminish in 

 size, the most posterior vertebrae being represented merely by 

 nearly cylindrical centra. 



There are twelve pairs of ribs, of which the first seven are known 

 as true ribs, i.e., are connected by their cartilaginous sternal ribs 

 with the sternum; while the remaining five, the so-called false or 

 floating ribs, are not directly connected with the sternum. All, 

 except the last four, bear two articular facets, one on the vertebral 

 extremity or cdpitulum^ and the other on a little elevation or tubercle 

 situated at a little distance from this the former for the bodies, 

 the latter for the transverse processes of the vertebrae. 



The sternum consists of six segments or sternelrce, the first 

 the manubrium sterni or presternum is larger than the rest, and 

 has a ventral keel. With the last is connected a rounded cartila- 

 ginous plate, the xiphisternum. 



The skull (Figs. 1082, 1083), if we leave the jaws out of account, 

 is not at all unlike that of the Pigeon in general shape. The 

 length is great as compared with either the breadth or the 

 depth ; the maxillary region, or region of the snout (corresponding 

 to the beak of the Pigeon) is long in proportion to the rest, the 

 orbits closely approximated, being separated only by a thin inter- 

 orbital partition, and the optic foramina united into one. But 

 certain important differences are to be recognised at once. One of 

 these is in the mode of union of the constituent bones. In the 

 Pigeon, as we have seen, long before maturity is attained, the bony 

 elements of the skull, originally distinct, become completely fused 

 together, so that their limits are no longer distinguishable. In the 

 Rabbit, on the other hand, such fusion between elements only takes 

 place in a few instances, the great majority of the bones remaining 

 distinct throughout life. The lines along which the edges of 

 contiguous bones are united the sutures as they are termed are 

 sometimes straight, sometimes wavy, sometimes zig-zagged, serra- 

 tions of the edges of the two bones interlocking ; in some cases the 

 edges of the bones are bevelled off and the bevelled edges overlap, 

 forming what is termed a squamous suture. 



Another conspicuous difference between the skull of the Rabbit 



