150 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



has been described in the birds ; but in mammals one frequently 

 exists, but here it is placed anterior to instead of ventral to the 

 sternum proper. Where best developed it is T-shaped ; and it 

 may be movably articulated to the sternum as in the mono- 

 tremes (Fig. I 58), or firmly united to it (marsupials). In cer- 

 tain rodents it becomes divided into three parts, while in the 

 primates it is reduced to the intermediate cartilages by which 

 the clavicles articulate with the sternum. The omosternum of 

 the anura was formerly regarded as an episternum, but it is 

 apparently truly sternal in nature. 



The Skull. The skeleton of the head, the skull, is a very 

 complicated structure ; and in it two regions may be recognized, 

 a cranium for the protection of the brain and sense organs 

 (eyes, nose, ears), and a visceral skeleton which forms the jaws, 

 and gives support to the visceral walls. In the beginning all 

 of these parts are outlined in cartilage ; and in marsipobranchs 

 and elasmobranchs they never pass beyond the cartilage stage, 

 although, as in some sharks, the outer portions of the cartilage 

 may be calcified 1 in the adult. In the higher groups this carti- 

 lage may be partially or almost completely converted into bone ; 

 and in all vertebrates above the elasmobranchs there are added 

 to those portions of the skull which are of cartilage origin 

 numerous other skeletal elements which are not preformed in 

 cartilage, but which arise as ossifications of membranes. It 

 therefore becomes necessary to distinguish in the higher verte- 

 brates between cartilage-bones and membrane-bones, but these 

 distinctions can be made only by tracing the development ; 

 there is nothing in the fully developed bone which will decide 

 the question. 



In the development of the cartilaginous cranium (chondro- 

 cranium) there occurs first the formation of a membranous cap- 

 sule, the primordial cranium, which encloses the brain and sense 

 organs. In no vertebrate have the details of this membranous 

 cranium been worked out. Later there is a chondrification of 

 this primordial cranium which proceeds from several distinct cen- 

 tres, which may be spoken of as the parachordals, otic capsules, 

 trabeculae, and nasal capsules. 



1 The distinction between calcined cartilage and bone is important. 



