THE SKULL 



53 



Fig. 29. Transverse section 

 of shark vertebra Ccartilage 

 stage), from young specimen 

 of Atlantic dogfish, Acanthias: 

 i, intercalary cartilage, com- 

 pleting arch; n, notochord; 

 V, body of vertebra. 



The Skull 



Composition of the Skull 

 The head skeleton of a mammal, usually 

 but inaptly called the skull, is a complex 

 of individual bones and cartilage, the 

 arrangement and functions of which may 

 be determined with a Httle effort. The 

 general disposition of the bone elements, 

 demonstrable in the rabbit or any mam- 

 mal is as indicated in Fig. 30. Briefly, 

 there is a linear series of basal segments, 

 comprising from behind forward basioccip- 

 ital, basisphenoid, presphenoid, and mes- 

 ethmoid. The three first-named form the 

 floor of the brain-case, while the meseth- 



moid forms the nasal septum. Associated with the basioccipital 

 are paired, lateral exoccipital bones, and a supraoccipital element, 

 together forming an occipital ring enclosing the aperture for trans- 

 mission of the spinal cord from the cranial cavity (Fig. 25). In 

 ancestral tetrapods, the basioccipital and exoccipital bones took 

 part in the formation of a single rounded condyle for articulation 

 with the first vertebra. In mammals, however, the posterior end 

 of the basioccipital has become reduced, leaving only the two 

 lateral components of the original condyle to make this articulation. 

 Hence the mammal has paired occipital condyles, borne upon the 

 exoccipital bones only and constituting a stronger joint with motion 

 practically restricted to the vertical plane. The basisphenoid and 

 presphenoid bear lateral expansions, respectively the greater and 

 lesser wings, or alisphenoids and orbitosphenoids, which assist in 

 the formation of the side walls of the brain-case. The bony capsule 

 (periotic bone) lodging the internal ear on either side is solidly 

 built into the lateral cranial wall between the exoccipital and the 

 alisphenoid, while further forward- the light scroll-Hke surfaces of 

 the ethmoid bone, or ethmoturbinal, representing the chief, or 

 olfactory portions of the nasal capsule, are attached on each side 

 of the base of the mesethmoid. The original extent of the nasal 

 capsule is, however, more nearly commensurate with the general 



