THE BEAIN. 265 



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 prosenceplialon 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 prosen- 



FiG. 57. — Soyllium 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, prosenceplialon. 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. MH, external or horizontal semi- 

 circular canal. ITC, anterior cardinal sinus. M"H, opening of hyoidean sinus 

 into anterior cardinal sinus. UL, nerve of the lateral line. H P, post-orbital 

 sinus. O, olfactory organ. OI, obliquus inferior. OS, obliquus superior. 

 BI, rectus internus. SB, 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 nei-ve. VII. B', buccal branch 

 of seventh nerve. VII. B, palatine branch of seventh nerve. VII. C, 

 hyoidean branch of seventh nerve. VII. D, ' prespiracular ' branch of seventh 

 nerve. VIII, auditory nerve. IX, ninth or glosso-pharyngeal nerve. X, 

 tenth or pneumogastric 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 pneumogastric nerve to fifth branchial cleft. 

 X. F, visceral branch of pneumogastric nerve. 



