GENERAL MORPHOLOGY OF THE NERVOUS SYSTEM. 31 



The forebrain of dipnoans and amphibians differs from the 

 compact type in selachians in two ways. First, the olfactory tract 

 is absent as an external feature and the bulb is connected directly 

 with the lateral lobe. Second, the lateral lobes are separated 

 from one another in front (Compare Fig. 8, B, D, and Fig. 9) the 

 median ventricle is still shorter and the lateral ventricles are 

 relatively still more important. The lateral lobes have complete 

 nervous walls and the tela chorioidea covers only the median 

 ventricle. In reptiles, birds and mammals the lateral lobes are 

 fundamentally of the amphibian type but become larger and greatly 

 modified in connection with the development of the cerebral hem- 

 ispheres. (See Chapter XVII.) 



In most vertebrates only one pair of nerves is connected with 

 the forebrain. These are the olfactory nerves which were men- 

 tioned above. The olfactory nerve always arises from the sense 

 cells of the nasal epithelium, has no ganglion in its course and 

 enters the olfactory bulb. In addition to the olfactory there is 

 found in many selachian and in some ganoid and dipnoan fishes 

 another pair of nerves connected with the forebrain, whose presence 

 is of the greatest importance in the study of the morphology of 

 the nervous system. This new nerve arises from the upper 

 or lower surface of the forebrain near the median line, bears 

 a ganglion which is often lodged in the angle between the 

 olfactory tract and the forebrain, passes forward along the olfac- 

 tory tract and over the olfactory bulb, divides into branches and 

 is distributed to the epithelium of the nasal sac (Fig. 2). This 

 nerve differs from the olfactory nerve in that its fibers are mye- 

 linated, that it has a ganglion and that it enters the forebrain 

 proper and not the olfactory bulb. Because of its attachment to the 

 front end of the brain, this nerve has been called the nervus term- 

 inalis. 



The figures in this chapter are intended rather to illustrate 

 the typical form of the vertebrate brain and to bring out features 

 of special morphological significance than to show the varying 

 forms of the brain in different classes. For this the reader should 

 refer to the figures in the larger text-books of zoology and com- 

 parative anatomy. 



