chap, ix.] VERTEBRAL COLUMN. 321 



of the cartilaginous discs that lie between the several 

 vertebrae, and, in the more erect forms, by its 

 sigmoid curvature ; the bony outgrowths of the 

 several parts may be elongated or broadened to serve 

 as larger areas of attachment for the muscles. 



The number of cervical vertebrae among the 

 Sauropsida varies with the length of the neck ; 

 the swan, for example, having as many as twenty- 

 five.* In all these hand-less Vertebrates the neck 

 is of great mobility ; freedom of movement to 

 the vertebrae on one another being allowed by the 

 already described saddle-shaped form of their centra. 

 In the Chelonia also, where much of the body is in- 

 vested in the firm carapace, the neck is very flexible, 

 and the shape of the centra varies greatly not only in 

 different species, but in the different cervical vertebrae 

 of the same animal ; the neural spines are never well 

 developed, and the head can be retracted or protruded. 

 The succeeding vertebras in the Chelonia have flattened 

 centra, but most are more remarkable for the possession 

 of a broad plate of bone, which is connected with the 

 apex of the neural arch, and forms one of the median 

 " neural plates " of the exoskeletal carapace the ver- 

 tebrae of the tail can be moved on one another, and 

 are, like most of the vertebrae of most Reptiles, procce- 

 lous. The most important exceptions to this law 

 have been already noted. 



While in the Amphibia only one vertebra enters 

 into relation with the ilium, or can be spoken of as 

 sacral, there appear to be two true sacral vertebrae in 

 the Saiiropsida. In Birds, where the whole support 

 of the body falls on the hind limbs, a number of pre- 

 sacral and post-sacral vertebrae fuse with the true 

 sacrals, to form a firm mass of attachment and sup- 

 port. Where the bird has, like the ostrich, to depend 



* Some of the extinct tlesiosauria had more than forty cervical 

 vertebrae. 



v 16 



