124 



THE HUMAN BODY 



the only portion of the cord where the gray substance is un- 

 covered by white. Running along the middle of the gray com- 

 missure, for the whole length of the cord, is a tiny channel, just 

 visible to the unaided eye; it is known as the central canal (canalis 

 centralis) . J^ 



The Brain (Fig. 60) is farlaj^er tkan the sp^pf cord and more 

 complex in structure. It wjjghs oirthe avera^about l,41^Hgrams 

 (50 ounces) in the adultale,/gmd aboutslfro grams gro ounces) 

 less in the female. Iiyfts simpler forms jttfe v)l(ftebfle brain con- 



Mo 



FIG. 61. The brain from the left side. Cb, the cerebral hemispheres forming 

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

 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 

 on each side are two large gray masses, the corpora striata and 

 optic thalami. The midbrain forms a connecting isthmus between 



