298 NERVOUS SYSTEM OF VERTEBRATES. 



fibers which constitute the anterior commissure have not been 

 definitely traced. 



This is the most primitive type of forebrain that has been 

 studied. It is concerned with olfactory impulses and possibly 

 with gustatory impulses which may reach the epistriatum by way 

 of the hypothalamus. It will be seen as the account proceeds 

 that the centers and fiber tracts which have been described are 

 preserved in higher vertebrates. 



THE SELACHIAN FOREBRAIN. The forebrain of the primitive 

 selachians (e.g. Heptanchus, Fig. 2) and of the Holocephali (Chi- 

 maera, Fig. 7) is rather slender and elongated. That of the 

 more specialized forms (e.g. Squalus acanthias, Figs, n and 147, 

 Scyllium, Raja, Torpedo, etc.) is much more massive and compact 

 in form. The difference is chiefly a difference in size due to the 

 greater development of certain parts of the brain in more specialized 

 selachians. In both, the walls of the median ventricle are essen- 

 tially as in cyclostomes except that the ventro- cephalic and the front 

 part of the lateral wall are enormously thickened. The foramen 

 of Monro leads laterally from the front end of the median ventricle 

 into the lateral ventricle which traverses the thick lateral mass 

 and continues forward through a more or less elongated olfactory 

 tract into the olfactory bulb (Fig. 8). This is the typical form 

 for the olfactory bulb in vertebrates. In cyclostomes the bulb 

 has been pushed out to the side and backward by pressure from 

 the large buccal funnel, and the side wall of the forebrain (olfactory 

 lobe) has been folded outward and backward so as to form a sort 

 of posterior horn to the lateral ventricle which is quite peculiar 

 to the cyclostomes. 



The great mass at the front of the brain is composed of two parts 

 the limits between which are readily seen in Figs, n and 147. 

 Above and in front of the foramen of Monro the median ventricle 

 is produced into a small pointed sac, the recessus neuroporicus. 

 This meets a fibrous strand from the pia which descends through 

 a narrow canal from the dorso-cephalic surface of the forebrain. 

 This canal marks the dorsal border of the lamina terminalis and 

 the mass lying in front of it is formed by a thickening of the lamina 

 terminalis and of the adjacent wall, between the bases of the two 



