CVa.s-.v 4. Reptilia. 



409 



In the adult, tte skeleton is only to a slight extent carti- 

 laginous ; it consists almost entirely of bone. The notochord has 

 usually disappeared ; only in the Geckos does it persist as a 

 continuous cord extending the whole length of the vertebral column.* 

 The centra usually articulate; they are generally proccelous ; 

 in the Crocodilia, there are cartilaginous discs between them.t 

 The vertebral column is usually divisible into more regions than 

 in the Amphibia: first, there is a variable number of cervical 

 vertebrse, without ribs or with short ones; then a number 

 provided with longer ribs, the thoracic vertebras; these are 

 often followed by several ribless lumbar vertebrae; then the 

 sacral vertebrse, usually two, to the transverse processes of 

 which the pelvis is attached (occasionally, especially in certain extinct 

 Reptilia, there is a larger numbers of sacrals) ; lastly, the caudal 

 vertebrae,! without ribs. In the Snakes, however, in consequence 

 •of the absence of limbs, these distinctions do not hold ; all the 

 cervical and dorsal vertebree, with the exception of the first, bear 

 well-developed ribs; there are no sacrals, and therefore trunk and 

 caudal vertebrae only can be distinguished. The first two cervicals, 

 the atlas and axis, are peculiar in form (Fig. 333) . The centrum 



"Fig. 333. Diagrammatic 

 transverse section of the 

 atlas of one of the higher 

 Vertebrata. 6 ^ arch of the 

 atlas, 1 odontoid process, 

 m bony plate, I ligament. — 

 ■Orig. 



Fig. 334. Diagram of 

 axis. 1 centrum of atlas, 

 2 centrum of axis, 62 ^^oTi 

 of axis. — Orig. 



Fig. 333. 



( I 



Fig. 334. 



k 



J 



•of the former is fused with the second, forming a process (the 

 odontoid) at its anterior end. The first vertebra is, therefore, 

 merely a bony ring formed by an arch, bridged below by a bony 



* In the young Lacerta, etc., considerable portions of tlie chorda dorsalis are 

 present in the centra, but they disappear later. 



fTransverse processes are especially well-developed in the CrooodUia ; 

 here tbey are large on most of the vertebraa, although elsewhere they are most 

 prominent in the taU. Frequently {e.g., in the Snakes) an unpaired process arises 

 from the ventral side of the centrum of many of the vertebrse, the ventral spine. 

 Definite articvdar processes (zygapophyses) are present ; in the Snakes and some 

 Lizards (Iguana), besides these, there arises from the anterior side of each 

 neural arch, a single process, with two articular facets fzygosphene), each of whioli 

 fits into a pit (zygantrum) in the preceding vertebra, and thus the connection is made 

 still firmer. 



J In most Lizards, the tail breaks with peculiar readiness ; this is correlated with 

 the fact that in the middle of each caudal centrum, there is an uncalcified transverse 

 •disc. After fracture, the tail grows again. 



