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A YEAR IX SCIENCE 



its average diameter is about three-fourths of an inch. 

 A number of large nerves, thirty-one pairs, arise from 

 the spinal cord. Their branches pass to all parts of 



Dorsal Root 



White.. Matter 



Fig. 179. Section of the spinal cord showing- the method of 

 origin of the spinal nerves by two roots. The arrows indicate the 

 direction in which the nerve impulses pass through these roots. 



the body. Each of these nerves has two roots. The 

 ventral root is composed of fibers which carry currents 

 out to the muscles and is, therefore, called the motor 

 root. The dorsal root carries currents from the organs 

 to the spinal cord. Consequently, we would assume 

 that if dorsal roots are in any way injured, a person 

 would lose the sensation of feeling in the part from 

 which these nerves led. Similarly, if the ventral roots 

 are disabled the power of motion would be lost. 

 Experiments on lower animals or observations in 

 human beings in which the cord has been injured, or 

 diseased, confirm these assumptions. 



The spinal cord is of great importance; first, because 

 it relieves the brain of a great deal of work; second, 

 because it is a passageway for conducting messages of 



