THE NERVOUS SYSTEM. 107 



CHAPTEE VIII. 



THE NERVOUS SYSTEM. 



1. The Organs of the Nervous System. In speaking 



of the nervous system, reference is made to the brain, spinal 

 cord, the nerves distributed to every part of the body, and to 

 ganglia, found in certain parts. The brain and spinal cord 

 are continuous through the large opening at the base of the 

 skull. They form the cerebro- spinal centre or axis. Their 

 structure is so soft and so easily crushed, it is essential they 

 should be well protected ; they are therefore inclosed in bony 

 cavities. They consist of two kinds of matter, a white fibrous 

 portion and a gray vascular portion. 



2. The Brain. The brain is a mass of white fibres, over- 

 laid with cells of gray matter, and lodged within the strong 

 bony walls of the skull. It consists of two parts, the cere- 

 brum, situated at the summit and in front, and the cerebellum, 

 placed below and behind. 



The weight of the brain ranges from forty to forty-seven 

 ounces in the female, and from forty-six to fifty-three ounces 

 in the male. The weight increases rapidly during early child- 

 hood. After the seventh year it grows less rapidly. From 

 sixteen to twenty the increase is still more slow. Between 

 thirty and forty the weight begins slowly to decrease. The 

 weight of the brain in man is greater than in any of the 

 lower animals, except the elephant and whale. As a general 

 rule, the size of the brain indicates the intellectual capacity 

 of the individual, but there are some striking exceptions. 

 The size of the cerebrum is a more reliable guide in deter- 

 mining the mental power, but even this is not always a 



