CHAP. XVIII.] THE NERVOUS SYSTEM. 207 



uous column, arid as forming the core of the central nervous 

 system, while around it are built up the great mass of the cere- 

 brum and the smaller mass of the cerebellum. This central 

 core is connected by various ties with the spinal cord, besides 

 being, as it were, a continuation of the gray matter in the 

 centre of the cord. It is also connected at its upper end, by 

 numberless fibres, to the gray matter on the surface of the cere- 

 brum. 



The different masses of gray matter being so closely asso- 

 ciated and connected with one another, it will be seen that it 

 becomes a very difficult matter to assign definite functions to 

 definite areas. Of late years, however, a great deal has been 

 accomplished in this direction, and it has become possible to 

 locate the centres that preside over the organs of speech, sight, 

 hearing, etc., and over the movement of muscles in any part 

 of the body. Owing to the crossing or decussation of fibres in 

 the medulla oblongata, injuries to nerve-centres on the right 

 side of the brain will affect the left side of the body, below the 

 decussation. 



The average weight of the brain in the male is 49|- oz. ; in 

 the female, 44 oz. It appears that the weight of the brain in- 

 creases rapidly up to the seventh year, more slowly to between 

 sixteen and twenty, and still more slowly to between thirty and 

 forty, when it reaches its maximum. Beyond this age the brain 

 diminishes slowly in weight, about an ounce every ten years. 

 The size of the brain bears a general relation to the capacity of 

 the individual. Cuvier's brain weighed rather more than 64 oz., 

 while the brain of an idiot seldom weighs more than 23 oz. The 

 number and depth of the cerebral convolutions also bear a close 

 relation to intellectual power ; babies and idiots have few and 

 shallow folds, while the brains of men of intellect are always 

 markedly convoluted. 



The cranial nerves. The cranial nerves, twelve in number on 

 each side, arise from the base of the brain and medulla oblon- 

 gata (iride Fig. 120), and pass out through openings in the base 

 of the skull. They are named numerically according to the 

 order in which they pass out of the skull. Other names are also 

 given to them derived from the parts to which they are dis- 

 tributed, or from their functions. Taken in their order from 

 before backwards, they are as follows : 



