74 



ZOOLOGY 



SECT. 



regions are to be distinguished : a posterior or occipital region, 

 formed from the parachordals, united or articulated with the 

 anterior end of the vertebral column, and presenting a large 

 aperture, the foramen magnum (Fig. 773, B, for. mag.), through 

 which the spinal cord becomes continuous with the brain ; an 

 auditory region formed by the two outstanding auditory capsules 

 (A, au. cp.) ; and a trabecular region, including all the rest. The 

 latter is again divisible into an interorbital region, between the 

 orbits or eye-sockets ; an olfactory region, constituted by the 

 olfactory capsules (olf. cp.), and by a median vertical plate, the 

 mesethmoid (B, m. eth.), which separates them from one another ; 

 and a pre-nasal region or rostrum (r) extending forwards from the 



B 



FIG. 773. A, diagram of cartilaginous skull from the left side ; B, cranium in sagittal section. 

 au. cp. auditory capsule ; b. br. 1 -5, basi-branchials ; b. cr. basis cranii ; b. hy. basi-hyal ; 

 c. br. cerato-branchial ; c. hy. cerato-hyal ; ep. br. epi-branchial ; ep. hy. epi-hyal ; fan. fon- 

 tanelle ; for. mag. foramen magnum; h. br. hypo-branchial; h. hy. hypo-hyal ; hy. m. 

 liyomandibular ; Ib. 14, labial cartilages ; mclc. c. Meckel's cartilage ; m. eth. mesethmoid ; 

 no. 1 10, foramina for cerebral nerves ; olf. cp. olfactory capsule ; pal. tju. palato-quadrate ; 

 ph. br. pharyngo-branchial ; r. rostrum ; s. t. pituitary fossa or sella turcica. 



mesethmoid and forming a more or less well-marked anterior 

 prolongation of the cranium. The cavity for the brain (B) extends 

 from the foramen magnum behind to the olfactory region in front ; 

 its floor, formed from the basal plate of the embryo, is called the 

 basis cranii (b. cr.) : its roof is always incomplete, there being one or 

 more apertures or fontanelles (fon.) closed only by membrane and 

 due to the imperfect union above of the side-walls. 



In the walls of the brain-case are apertures or foramina for 

 the passage outwards of the cerebral nerves (vide p. 100). The 

 most important of these are the olfactory foramina (nv. 1) for the 

 nerves of smell, situated at the anterior end of the cerebral cavity, 



