THE NERVOUS SYSTEM. 



127 



The rudimentary condition of the olfactory lobes, (b). A perfectly denned 

 fissure of Sylvius, (c). A posterior lobe completely covering the cerebel- 

 lum, (d). The presence of posterior cornua in the lateral ventricles 

 (Gratiolet). 



The most distinctive points in the human brain, as contrasted with 

 that of apes, are: (1). The much greater size and weight of the whole 

 brain. The brain of a full-grown gorilla weighs only about 15 oz., which 

 is less than ^ the weight of the human adult male brain, and barely 

 exceeds that of the human infant at birth. (2). The much greater com- 

 plexity of the convolutions, especially the existence in the human brain of 

 tertiary convolutions in the sides of the fissures. (3). The greater relative 



FIG. 340. Brain of the Orang, % natural size, showing the arrangement of the convolutions. Sy, 

 fissure of Sylvius; R, fissure of Rolando; EP, external perpendicular fissure; Olf, olfactory lobe; 

 Cft, cerebellum ; PP, pons Varolii ; M O, medulla oblongata. As contrasted with the human brain, 

 the frontal lobe is short and small relatively, the fissure of Sylvius is oblique, the temporo-sphenoidal 

 lobe very prominent, and the external perpendicular fissure very well marked. (Gratiolet.) 



size and complexity, and the blunted quadrangular contour of the frontal 

 lobes in man, which are relatively both broader, longer, and higher, than 

 in apes. In apes the frontal lobes project keel-like (rostrum) between the 

 olfactory bulbs. (4). The much greater prominence of the temporo- 

 sphenoidal lobe in apes. (5). The fissure of Sylvius is nearly horizontal 

 in man, while in apes it slants considerably upward. (6). The distinct- 

 ness of the external perpendicular fissure, which in apes is a well-defined 

 almost vertical "slash," while in man it is almost obscured by the an- 

 nectent gyri (Rolleston). 



Most of the above points are shown in the accompanying figure of the 

 brain of the Orang. 



Functions. (1.) The Cerebral hemispheres are the organs by which 

 are perceived those clear and more impressive sensations which can be 

 retained, and regarding which we can judge. (2.) The Cerebrum is the 

 organ of the will in so far at least as each act of the will requires a de- 

 liberate, however quick determination. (3.) It is the means of retaining 



