Our Nervous System 431 



running, using a sewing machine, playing a piano and in doing 

 all kinds of manual work. 



In the study of the life processes of your body you must have 

 noticed the cooperation among its internal organs. It is necessary 

 that the activities of the various organs be so directed that perfect 

 teamwork is secured. To accomplish this there must be a directing 

 center, connected with all parts of the body, to receive informa- 

 tion and to issue orders. This center is the brain and its connec- 

 tions are made through the spinal cord and the nerves. Together 

 they form the nervous system. 



Cellular Structure of the Nervous System. In the brain 

 and the spinal cord two kinds of nerve tissue are found, one com- 

 posed of gray matter, the other of white matter. The gray matter 

 contains a vast number of cells, somewhat irregular in shape, from 

 most of which minute branches called dendrites are given off. 

 These are the parts that enable the nerve cells to communicate 

 with each other. The white matter is made up largely of nerve 

 fibers. Each fiber contains a central part called the axis cylinder, 

 which is the essential part of the nerve, since it transmits 

 the nerve messages to and from the brain. Around the cylin- 

 der is a sheath and outside of this a thin covering of connec- 

 tive tissue. The axis cylinder of each nerve fiber is found to 

 connect, in one place or another, with at least one central cell 

 of gray matter. Each cell and its branches, including the axis 

 cylinder, form one unit of the nervous system called a neuron, 

 or nerve. 



Parts of the Nervous System. The principle divisions of 

 the nervous system are the brain, the nerves and the spinal cord. 



The Brain. The main parts of the brain are the cerebrum, 

 the cerebellum and the medulla. 



The Cerebrum. The cerebrum is the seat of thought. It is 

 composed of two hemispheres closely connected by nerve fibers. 

 It forms over eighty per cent of the brain, lies over and above 

 the other parts, and is composed of a mass of white nerve tissue 

 covered with a thin layer of gray nerve tissue about one-eighth of 

 an inch thick containing many large cells. The surface of the 

 cerebrum is formed into folds, or convolutions, which greatly 



