THE BRAIN. 273 



The brain nearly fills the cranial cavity : it is narrow 

 between the orbits, but expands considerably in front of 

 them. As seen from above, the following parts are visible 

 in order from before backwards. 



1. The prosencephalon is the most anterior and' the largest 



division of the brain. It is a smooth somewhat 

 globular mass, lying in the expanded anterior part 

 of the cranial cavity, between the olfactory capsules 

 and the orbits. It is marked in front by a shallow 

 median groove, and is sharply separated behind from 

 the succeeding part of the brain. It corresponds to 

 the cerebral hemispheres of higher vertebrates. 



2. The olfactory lobes are a pair of large oval masses, 



arising by stout stalks from the sides of the 

 prosencephalon, their distal surfaces being closely 

 applied to the olfactory capsules. 



3. The thalamencephalon is the narrow portion of the brain 



immediately behind the prosencephalon ; its roof is 

 very thin, so that the cavity, or third ventricle, 

 can be seen through it. From the hinder part of the 

 roof arises the pineal body, a slender tubular process 

 which extends forwards over the prosencephalon, 



Fig. 59. Scyllium catulus. Dissection of the brain and cranial nerves 

 from the dorsal surface. The right eye has been removed. The 

 cut surfaces of the cartilaginous skull and vertebral column are 

 dotted. About two-thirds the natural size. (a. m. m.) 



B, prosencephalon. BB, pineal body. BC, cerebellum. BO, olfactory 

 lobe. BS, spinal cord. BT, thalamencephalon. CA, auditory capsule. CO, 

 olfactory capsule. E, eye. F.1-F.5, first to fifth branchial clefts. H.1-H-4, 

 first to fourth branchial arches. M H , external or horizontal semi-circular canal . 

 NC, anterior cardial sinus. N H , opening of hyoidean sinus into anterior cardinal 

 sinus. N.L, nerve ofthe lateral line. N P, post-orbital sinus. O, olfactory organ. 

 O I, obliquus inferior. OS, obliquus superior. R|, rectus internus. SD, dorsal 

 root of spinal nerve. SP, hyomandibular cleft or spiracle. SV, ventral root of 

 spinal nerve. 



II, optic nerve. Ill, third nerve, iv, fourth nerve. V, VII. A, ophthalmic 

 branches of fifth and seventh nerves, v. B, maxillary branch of fifth nerve. 

 V. 0, mandibular branch of fifth nerve. VI I . B, palatine branch of seventh nerve. 

 VII. B 1 , buccal branch of seventh nerve. VII. 0, hyoidean branch of seventh 

 nerve. VI I. D, ' pre-spiracular ' branch of seventh nerve. VIII, auditory nerve. 

 IX, ninth or glossopharyngeal nerve. X, tenth or penumogastric nerve. X. A, 

 branch of pneumogastric nerve to second branchial cleft. X. B, branch of 

 pneumogastric nerve to third branchial cleft. X. D, branch of pneumogastrio 

 nerve to fifth branchial cleft. X. F, visceral branch of pneumogastric nerve. 



