56 



Anatomy of the Rabbit. 



CHONDROCRANIUM 

 AND OSTEOCRANIUM. 



The skull consists primarily in the embryo of a cartilage trough, 

 the extent of which is roughly definable as the area occupied by the 

 occipital, anterior and posterior sphenoidal, and ethmoidal portions 

 (Fig. 29.) As a carcilage skull it is designated as the chondro- 



cranium, and after its conversion into 

 bone as the osteocranium. It is no 

 more than an enclosure for the brain, 

 except that it has associated with it the cartilage capsules of the 

 nasal, visual, and auditory organs, and, in the case of the first 

 and last of these, the capsules are incorporated with the skull 

 proper. Thus, the primary skull is designated as the neuro- 

 cranium or cerebral cranium, to distinguish it from a second 



portion of the head skel- 

 eton, the splanchno- 

 cranium or visceral cran- 

 ium, which includes the 

 series of visceral arches 

 suspended from the ventral 

 surface of the neurocran- 

 ium. The addition to the 

 primary head skeleton of a 

 large number of membrane 

 bones results in more or 

 less confusion of the 

 original divisions, since the 

 membrane portions of the visceral cranium are, with the exception 

 of the mandible, united by suture with those of the cerebral cranium, 

 while the true cartilage or cartilage bone portions of the former, 

 occurring as the auditory ossicles, the hyoid and larnyx (in part), 

 although highly modified, remain in a more or less independent 

 relation. 



The appearance of the mammalian skull during the later stages 

 of foetal development is most striking, the cartilage mass of the 

 chondrocranium, and the bones ossifying in its interior forming a 

 foundation basal mass, from which are suspended elements of the 

 same nature, principally auditory and hyoid, in a somewhat arch or 

 rod like form. The auditory arch is formed by the two more 

 lateral bones of the auditory chain, incus and malleus, of which the 



Fig. 30. The chondrocranium and visceral arches 

 of the Atlantic dogfish, Acanthias: ca, auditory 

 capsule; ch, chondrocranium; en, nasal capsule; 

 h,h', dorsal and ventral segments of hyoid arch 

 (II); i, intercalary cartilage of vertebral column; 

 m,m', dorsal and vential poitions of mandibular 

 arch (II), functional upper and lower jaws; malleus 

 and incus of mammalian ear; or, orbit, depression 

 for optic capsule; v, vertebra; 1-5, btanchial arches. 



