THE ELASMOBRANCH FISHES 235 



to form a mixed nerve trunk. The brain, like that of the 

 frog, consists of a fore-brain, mid-brain, and hind-brain. The 

 fore-brain is divided into cerebral hemispheres (prosencephalon) 

 and thalamencephalon, the mid-brain consists of the corpora 

 bigemina (mesencephalon), and the hind-brain is divided into 

 the cerebellum (metencephalon) in front and above, and the 

 medulla oblongata (myelencephalon) behind and below. The 

 shape and relative positions of these parts, as seen from the 

 dorsal surface, are shown in fig. 57, and it can readily be seen 

 that, though the general plan is the same, the details differ 

 from those of the frog. 



The cerebral hemispheres form the extreme anterior end of 

 the brain, and at first sight look like a single globular mass, 

 but closer inspection shows that the mass is divided nearly 

 into two by a fissure extending inwards from its anterior end. 

 From the sides of the cerebral hemispheres the olfactory 

 lobes arise as a pair of stout stalks which run outwards 

 and forwards and expand on either side into a triangular 

 nervous mass closely applied to the posterior face of the 

 olfactory capsule. Posteriorly, the cerebral hemispheres are 

 continued into the narrow thalamencephalon. This, the 

 posterior division of the fore-brain, contains a considerable 

 cavity, the third ventricle, which is roofed in above by a very 

 thin membrane. The hindmost part of this roof is produced 

 in the middle line into a slender tube which runs forward 

 over the cerebral hemispheres, and ends in a small dilatation 

 attached to the lower side of the membrane covering the 

 anterior fontanelle of the skull. The stalk and dilatation 

 together constitute the pineal outgrowth. The floor of the 

 thalamencephalon is also thin, and is produced into a hollow 

 finger-shaped process, the infundibulum, the proximal half of 

 which is directed backwards, while the distal half is bent 

 forward at a sharp angle so as to underlie the proximal half. 

 Close to their union with the thalamencephalon, the walls of 

 the proximal moiety of the infundibulum are expanded to 

 form a pair of conspicuous rather thick-walled sacs, known as 

 the lobi inferiores, and the infundibular walls are further 

 produced at the flexure into a pair of thin-walled lateral sacs, 

 the sacci vasculosi. The pituitary body is a very small tube 

 lying on the under side of the distal half of the infundibulum. 

 It is formed in embryonic life as an outgrowth of the dorsal 



