CHAPTER XXXV 

 THE GOVERNOR OF THE BODY 



207. Need of a Governor. The power of motion is inherent 

 in all protoplasm, but when this protoplasm is arranged in 

 tissues consisting of millions of cells, it is impossible to con- 

 ceive of their working in harmony without some kind of a 

 directing force. In addition, there must be a motor impulse 

 to cause the cells to work at all. Moreover, since the motor 

 impulse comes from within, there must be some means of 

 carrying sensations from the outside. The part of the body 

 charged with this task of direction is the nervous system, 

 which consists of the brain, the spinal cord, the nerves, and 

 the end organs. The principal parts of this system are care- 

 fully preserved from injury. The brain is enclosed in a bony 

 box, the cranium, which forms part of the skull, and is pro- 

 tected by three membranes ; the spinal cord runs down through 

 a series of bony arches formed by backward projections of the 

 vertebrae and the large nerves are deep in the flesh, close to the 

 bones, with only their finer divisions coming to the surface. 

 There are twelve pairs of these nerves given off by the brain 

 and thirty-one given off by the spinal cord. Each consists of 

 a tract over which sensations travel to the brain and a tract 

 over which motor impulses travel from the brain to the 

 muscles, glands, and other end organs. These impulses are 

 similar to electricity in their manifestations and travel along 

 the nerves at the rate of more than 100 feet a second. New 

 impressions may originate in the brain and be carried out by 

 impulses sent to the cells in various parts of the body, but 

 usually some disturbance from outside stimulates the sensitive 



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