166 



THE HUMAN BODY. 



It supplies the lower part of the trunk, the buttocks, the 

 front of the thigh, and inner side of the leg. 



The sacral plexus is formed by the anterior primary 

 branches of the fifth lumbar and the first four sacral nerves, 

 which unite in one great cord and so form the sciatic nerve, 

 which is the largest in the Body and, running down the 

 back of the thigh, ends in branches for the lower limb. The 

 roots of the trunks which form the sacral plexus arise from 

 the lumber enlargement of the cord. 



The Brain (Fig. 73) is far larger than the spinal cord 

 and more complex in structure. It weighs on the average 



FIG 73. Diagram illustrating the general relationships of the parts of the brain. 

 A, fore-brain; b. mid-brain ; B, cerebellum; (7, pons Varolii ; D, medulla oblon- 

 gata ; B, C, and D together constitute the hind-brain. 



about 1415 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 consists of three masses, each with 

 subsidiary parts, following one another in series from before 

 back, and known as the fore-brain, mid-brain, and hind- 

 brain respectively. In man the fore-brain, A, weighing 

 about 1245 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 

 another by a deep median fissure, and known as the cerebral 

 hemispheres. The immense proportionate size of these is 

 very characteristic of the human brain. Beneath each cere- 



