56 



ANATOMY OF THE RABBIT 



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 neuro- 

 cranium. The addition to the primary head skeleton of a large 

 number of membrane bones results in more or less obscuring 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 cra- 

 nium, while the true cartilage 

 or cartilage bone portions of 

 the former, occurring as the 

 auditory ossicles, the hyoid, 

 and the larynx (in part), 

 although highly modified, re- 

 main in a more or less in- 

 dependent relation. 



The components are strik- 

 ingly distinct in the mam- 

 malian skull during the later 

 stages of foetal development, 

 the cartilage of the chondrocranium and the bones ossifying in its 

 interior forming a basal mass, from which, as a foundation, are sus- 

 pended 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 incus is attached to the skull, while the mal- 

 leus is extended as the cartilage of Meckel almost the whole 

 length of the inner surface of the mandible. The bulk of the 

 skull is formed already by the surface elements distributed in 

 the characteristic fashion, but as yet only loosely associated (cf. 

 Fig. 31). 



Head Skeleton of Lower Vertebrates 



This condition of the developing skull in a mammal finds its 

 explanation far back in the history of the vertebrates and is made 

 clear only by the study of some one of the lower fishes such as 

 shark or sturgeon. In a shark (Fig. 32) the entire internal skeleton 

 is formed in cartilage which persists throughout life. The principal 



Fig. 32. The chondrocranium and visceral 

 arches of the Atlantic dogfish, Acantliias: ca, 

 auditory capsule ; ch, chondrocranium ; en, nasal 

 capsule; h,h', dorsal and ventral segments of 

 hj'oid arch (II); i, intercalary cartilage of 

 vertebral column; m,m', dorsal and ventral por- 

 tions of mandibular arch (I), functional upper 

 and lower jaws; malleus and incus of mam- 

 malian ear; or, orbit, depression for optic capsule; 

 V, vertebra; 1-5, branchial arches. 



