56 ANATOMY OF THE RABBIT. 
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 cartilage skull it is designated as the chondro- 
CHONDROCRANIUM cranium, and after its conversion into 
AND OSTEOCRANIUM. 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, io 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- 

Fic. 30. Thechondrocranium and visceral arches 1um. The addition to the 
of the Atlantic dogfish, Acanthias: ca, auditory : e Ss 
capsule; ch, chondrocranium; cn, nasal capsule; primary head skeleton of a 
h,h’, dorsal and ventral segments of hyoid arch : rR 
(II); i, intercalary cartilage of vertebral column; large number of me mbrane 
m,m’, dorsal and ventral poitions of mandibular 2 oe sates 
arch (II), functional upper and lower jaws; malleus bones results Im more Or 
and incus of mammalian ear; or, orbit, depression 
for optic capsule; v, vertebra; I-5, branchial arches. 
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 
