2 6 THE NERVOUS SYSTEM 



indicated in Fig. 7, C, D. In the selachians, as illustrated in Figs. 8, 9, 10, 

 and 11, the evagination has progressed further than in cyclostomes. Still further 

 progress in this direction has been made by the. amphibians, the cerebral hemi- 

 spheres of which have reached about the stage of development indicated in Fig. 

 7, E, F, G. Here the entire lateral wall, including the pallium and corpus 

 striatum, has been evaginated in the formation of the cerebral hemisphere. 



The Brain Ventricles. The portions of the original cavity of the neural tube 

 which are contained within the evaginated cerebral hemispheres are known as 

 the lateral ventricles (Fig. 7, G). These paired ventricles communicate with the 

 median prosencephalic cavity by openings known as the interventricular foram- 

 ina. This median cavity, called the third ventricle, represents for the most 

 part the cavity of the diencephalon, but its rostral part, bounded by the lamina 

 terminalis, belongs to the telencephalon. It will be seen by a study of the 

 accompanying diagrams that this lamina also belongs to the telencephalon and 

 represents in a certain sense the rostral end of the brain. Its position should 

 be carefully noted in each of the diagrams. The cavity of the rhombencephalon 

 is known as the fourth ventricle and that of the mesencephalon as the cerebral 

 aqueduct. The latter connects the third and fourth ventricles. It will help us 

 to understand the morphology of the vertebrate brain if we now consider the 

 shape and arrangement of the various parts of a simple brain like that of the 



dogfish. 



THE BRAIN OF THE DOGFISH SQUALUS ACANTHIAS 



The telencephalon of the selachian brain is evaginated to form a pair of 

 laterally placed masses, the cerebral hemispheres, and in this respect is at a stage 

 of development not far removed from that represented in diagrams E, F, and G 

 of Fig. 7. The long axis of the brain is almost straight; and this freedom from 

 ventrodorsal curvatures makes it especially easy to recognize the various funda- 

 mental divisions already enumerated and to understand their relationship. 



The medulla oblongata, which together with the cerebellum forms the rhom- 

 bencephalon, is continuous at the caudal extremity with the cylindric spinal 

 cord, and within it the central canal of the spinal cord opens out into the fourth 

 ventricle (Fig. 8). The medulla, which has somewhat the shape of a trun- 

 cated cone, is considerably larger than the cord, but decreases in size as it is 

 traced backward toward their point of junction. In the mammal a conspicuous 

 transverse bundle of fibers, associated with the cerebellum, is found on the 

 ventral and lateral aspects of that part of the medulla which belongs to the 

 metencephalon and is known as the pons. But in the fish it is customary to 



