xiii PHYLUM CHORDATA 69 



centra. In front of the parachordals are developed a pair of curved 

 cartilaginous rods, the trabeculce (tr), which underlie the anterior 

 part of the brain, as the parachordals underlie its posterior part : 

 their hinder ends diverge so as to embrace the pituitary body (pty) 

 already referred to. Cartilaginous investments are also formed 

 around the organs of the three higher senses : a pair of olfactory 

 capsules round the organs of smell, one of optic capsules round the 

 organs of sight, and one of auditory capsules (au. c.) round the 

 organs of hearing. The optic capsule, which may be either fibrous, 

 or cartilaginous, remains free from the remaining elements of the 

 skull in accordance with the mobility of the eye ; it constitutes, in 

 fact, the sclerotic or outer coat of that organ. The olfactory capsules 

 are usually formed as outgrowths of the trabeculse, and are therefore 

 continuous with those structures from the first. The auditory cap- 

 sules are in some cases formed as outgrowths of the parachordals, 

 in others arise as independent cartilages, each of which, however, 

 soon unites with the parachordal of its own side. As development 

 goes on, the trabecuhe and parachordals become fused into a single 

 basal plate (Fig. 719, B, b. cr.) 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. 719, B, for. mag.), through 

 which the spinal cord becomes continuous with the brain. ; an 

 auditory region formed by the two outstanding auditory capsules 

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

 is again divisible into an inter-orbital region, between the orbits 

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

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

 (B. ms. eth.), which separates them from one another ; and a pre- 

 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 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 infra). The 

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



