The Nervous System. 



71 



organ in a mammal may be explained by reference to the general 

 plan as indicated in Fig. 39, which is based upon general features 

 of form in vertebrates and upon embryctnic development. For 

 comparison in the gross the brain of the frog (Fig. 38) offers one 

 of the best examples. 



The brain as first formed in the embryo appears as an anterior 

 expanded portion of the neural tube, or rather as three expansions 

 arranged in a linear series. They are described as the primary 

 cerebral vesicles; or, as primary divisions of the future brain, 

 they are designated in anteroposterior order as 

 the prosencephalon, mesencephalon, and 

 rhombencephalon. 



The first of the primary divisions, the pro- 

 sencephalon, or primary fore-brain, becomes 

 divided during development into two portions, 

 namely, an anterior portion, the end-brain or 

 telencephalon, which is largely a paired struc- 

 ture, and a second portion, unpaired, the dience- 

 phalon, or inter-brain. The larger paired 

 portion of the telencephalon is the basis of the 

 cerebral hemispheres. It contains, as divisions 

 of the primary cavity, a pair of cavities, the 

 lateral ventricles. The anterior portion of the 

 telencephalon, moreover, becomes differentiated, 

 so that a small terminal olfactory segment, the 

 rhinencephalon, is more or less perfectly 

 marked off from the rest. In the mammalian brain this part 

 is chiefly identifiable as the paired olfactory bulb, the latter 

 being the anterior portion of the olfactory lobe or olfactory brain, 

 and containing in its interior an extension of the lateral ventricle. 



The unpaired portion of the prosencephalon is considered as 

 belonging in part to the telencephalon and in part to the dience- 

 phalon. Its cavity, the third ventricle, is connected with the 

 lateral ventricles through the interventricular foramen. Its 

 anterior wall is formed by a transverse connection of the cerebral 

 hemispheres, the lamina terminalis. In all vertebrates this 

 portion of the brain is remarkable for the manner in which its wall 

 is differentiated. The ventral portion extends downward as a 



Fig. 38. The brain 

 of the frog from 

 the dorsal surface, 

 c, cerebellum; d, 

 diencephalon ; f v, 

 fourth ventricle; h, 

 cerebral hemisphere 



0, olfactory lobe; 



01, optic lobe. 



