The Skeletal System. 31 



column presents marked dorsolumbar and cervical flexures. The axis 

 of the skull proper, the basicranial axis, identifiable as a line passing 

 through the centres of the basioccipital, basisphenoid and presphenoid 

 bones, may differ by a considerable angle from that of the related cervical 

 vertebrae. Further, the basicranial line, if continued forward in the 

 rabbit, would pass through the dorsal wall of the skull immediately in 

 front of the orbits (See Plate II, or Fig. 33 of the divided skull).' It 

 accordingly differs by a considerable angle from the basal line of the face, 

 or basifacial axis. 



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 

 (cf. Plates III-V). As a cartilage 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 skeleton, the splanchnocranium or visceral cranium, which includes 

 the series of visceral arches suspended from the ventral surface of the 

 neurocranium. 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 auditorv ossicles, the hyoid and larynx 

 (in part) (Plate II), although highly modified, remain in a more or less 

 independent relation. 



The skull, or cranium — using that term in a general sense — may be 

 described as consisting of a cranial portion (the cranium proper) and of 

 a facial portion, the latter including as visceral structures the upper jaw 

 and the mandible, and as cerebral structures the parts of the turbinated 

 bones and the associated secondary respiratory tract formed by the 

 nasal fossae. In a mammal the investing membrane bones of the face 

 are largely associated with a great extension of the nasal capsules, or that 

 portion of the primary skull from which the turbinated cartilages are 

 formed. Thus, as illustrated in the transverse section of Plate III, the 

 nasoturbinals and maxilloturbinals are formed on lateral extensions of 

 the primary septum (mesethmoid cartilage), but are supported more 

 directly, and in the adult wholly, by the investing membrane bones 

 (nasals, maxillae and premaxillae). 



The elements of the head skeleton may be classified as follows: 



1. The CEREBRAL CRANIUM (cranium cerebrale or neurocranium), 

 including ; 



(a) The primary cartilage skull (chondrocranium), enclosing 

 the brain, and containing in its wall the olfactory and 

 auditory capsules (embryonic) ; 



