Chap. VIII.] THE SKELETON. 163 



hinder portien of which is basi-branchial in origin, and repre- 

 sents the fused basal elements of these branchial arches. 



So far all the bones of the skull, with the exception of the 

 palatines and pterygoids in some forms (e.g. fowl), are cartilage 

 bones, ossification taking place within a preformed mass of 

 cartilage. All the other bones are splint bones, ossification 

 taking place in membrane without the intervention of cartilage. 

 Such are the parietals, frontals, lachrymals, nasals, pre-maxillae, 

 maxillae, vomers, squamosals, jugals, quadrato-jugals (in the fowl, 

 and their anterior portion in the frog) ; dentary, angular, supra- 

 angular, and splenial, in the lower jaw ; and para-sphenoid at 

 the base of the skull. This is a single bone in the frog with 

 rostral and basi-temporal divisions. In the fowl the rostrum 

 and basi-temporal form distinct ossifications, and in the rabbit 

 the para-spherioidal rostrum is entirely suppressed, and only 

 minute rudiments [larger in the guinea pig] of the basi-temporal 

 guard have been discovered. The question of the homology of 

 the basi-temporal, however, is one that has received more than 

 one answer. 



The Vertebral Column. (1.) In the Rabbit. The vertebral 

 column of the rabbit has some 45 vertebrae. They are, reckoning 

 from before backwards, atlas, axis, 5 more cervical; 12 thoracic, 

 with free ribs; 7 lumbar; 4 sacral; about 15 or 16 caudal. 



We may take the fourth thoracic vertebra for illustration 

 (Fig. 53.) Below there is a solid mass of bone, the body or 

 centrum (c.), the ends of which are flattened and vertical. From 

 this springs the neural arch (n.) through which runs the spinal 

 cord. Above the neural arch rises the neural spine (n. s.). At 

 its sides project transverse processes (tr.) which bear at their ends 

 tubercular facets (./.), with which the tubercular processes of the 

 ribs articulate. If the third and fifth be placed in position 

 before and behind it, it will be found to articulate with them by 

 means of little bony shelves in front and behind. Those in front 

 (prezygapophyses, pr. z.) face upwards and outwards ; those behind 

 (postzygapophyseSy pt. z.) face downwards and inwards. When the 

 vertebrae are thus placed together a lateral space is seen between 



