30 NERVOUS SYSTEM OF VERTEBRATES. 



or folded in different vertebrates. This is properly called the 

 ■paraphysis and is not homologous with the structure of the same 

 name in mammals mentioned above (Figs, ii, 36, 150, 152, 158). 

 The latero- ventral walls of the forebrain are thick and are loosely 

 spoken of as the corpora striata. The thin portion connecting these 

 in the mid-ventral line is known as the lamina terminalis and is 

 thickened at one place by the fibers of the anterior commissure. 

 In many other selachians (e.g. Squalus, Fig. 11, ScyUium, Raja) 

 the forebrain is shorter and more compact and massive. The 

 olfactory bulbs are larger and the olfactory tracts usually shorter. 

 The lateral lobes are shorter, thicker, and more rounded. The 



, ^ ^^v^^.^^ Epiphysis Tectum mesencephali Cerebellum 



Olfactory lobe' n \ 



L. inferior- 



Fig. 12. — A sketch of the brain of a cyclostome fish, Lampetra Wilderi, as seen 

 from the left side. d. i, first dorsal spinal nerve; N. I. L, roots of the lateral line 

 nerves; Vm., two motor roots of the trigeminus, the smaller of which innervates 

 an eye-muscle; Vs., sensory root of the trigeminus. 



brain is wider between the olfactory tracts and the massive nervous 

 structure extends up on the dorsal surface farther, so that the 

 choroid roof is shorter than in Heptanchus. So too, the median 

 ventricle is shorter and the lateral ventricle is relatively more 

 important. (Compare Figs. 2, 11.) 



The cyclostome forebrain (Fig. 12), although outwardly bearing 

 a resemblance to the massive selachian type, in reality owes its form 

 to the pressure which is exerted upon it by the great buccal funnel 

 in front. The forebrain of ganoids and bony fishes resembles 

 the elongated type of selachian brain, but is more slender and is 

 simpler because the olfactory structures are less highly developed. 

 The tela chorioidea is more extensive and the lateral lobes are 

 smaller and more compact. The olfactory tract varies in length. 

 The lamina terminaHs is nearly horizontal (Figs. 148, 139). 



