76 ZOOLOGY SECT. 



, in some cases arise as outgrowths of the parachordals, in others 

 as independent cartilages, each of which, however, soon unites 

 with the parachordal of its own side. As development goes on, 

 the trabeculae and parachordals become fused into a single basal 

 plate (Fig. 764, B, b. or.) underlying the brain: the skull-floor 

 thus formed gives off vertical up-growths on each side which finally 

 close in above to a greater or less extent, and so give rise to a more 

 or less complete cranium or brain-case enclosing the brain and the 

 organs of smell and hearing, and furnishing open cavities or orbits 

 for the eyes. 



In the continuous solid cranial box thus formed certain definite 

 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. 764, 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 trabecidar 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, p re- 

 nasal region or rostrum (r) extending forwards from the meseth- 

 moid 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 fontanclles (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 infra). 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, 

 one on each side of the mesethmoid ; the optic foramina (nv. 2) 

 for the nerves of sight, in the interorbital region ; the trigeminal 

 foramina (nv. 5) for the fifth nerves, just in front of the auditory 

 capsule ; the auditory foramina (nv. 8) for the nerves of hearing, 

 in the inner wall of the auditory capsules ; and the vagus foramina 

 (Nv. 10) for the tenth nerves, immediately posterior to the auditory 

 capsules. 



In addition to the elements of the brain-case parachordals, 

 trabeculae, and auditory capsules there enter into the composition 

 of the skull another set of elements called visceral bars. These are 

 cartilaginous rods formed in the walls of the pharynx between the 

 gill-slits, and thus encircling the pharynx like a series of paired 

 half-hoops (Fig. 760, B, us. b.). The corresponding right and left 



