ANATOMY OF THE NERVOUS SYSTEM 145 



line at the bottom of the ventral fissure; this forms the ventral white 

 commissure. There is no dorsal white commissure, the bottom of 

 the dorsal fissure being the only portion of the cord where the gray 

 substance is uncovered by white. Running along the middle of 

 the gray commissure, for the whole length of the cord, is the central 

 canal, previously described. It is a tiny channel, just visible to the 

 unaided eye. 



The Brain (Fig. 58) is far larger than the spinal cord and more 

 complex in structure. It weighs on the average about 1,415 grams 

 (50 ounces) in the adult male, and about 155 grams (5.5 ounces) 

 less in the female. In its simpler forms the vertebrate brain con- 



Cb 



Po 



FIG. 59. The brain from the left side. Cb, the cerebral hemispheres forming 

 the main bulk of the fore-brain; Cbl, the cerebellum; Mo, the medulla oblongata; 

 P, the pons Varolii; *, the fissure of Sylvius; Ro, the fissure of Rolando; Po, the 

 Parieto-occipital fissure. 



sists of three masses, each with subsidiary parts, following one 

 another in series from before back, and known as the fore-brain, 

 midbrain, and hind-brain respectively. In man the fore-brain, 

 A, weighing about 1,245 grams (44 ounces), is much larger than 

 all the rest put together and laps over them behind. It consists 

 mainly of two large convoluted masses, separated from one an- 

 other by a deep median fissure, and known as the cerebral hemi- 

 spheres. The immense proportionate size of these is very char- 

 acteristic of the human brain. Beneath each cerebral hemisphere 

 is an olfactory lobe, inconspicuous in man but in many animals 

 larger than the cerebral hemispheres. Buried in the fore-brain 



