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ADVANCED PHYSIOLOGY 



posterior roots of all the spinal nerves of the body, therefore, 

 is to carry sensory messages or impulses which are always 

 passing into the cord or brain, and are frequently called 

 afferent impulses, and the nerves concerned, afferent nerves. 

 If, on the other hand, the ventral or anterior roots are cut 

 and the posterior left intact, the animal so injured is unable 

 to move its leg, but can feel perfectly anything in contact 

 with it. From this, one decides that the impulses from the 



brain or cord which go 

 to the muscles of the leg 

 leave the cord by the 

 anterior roots. The ante- 

 rior roots of all spinal 

 nerves, then, carry motor 

 impulses. Motor im- 

 pulses always pass from 

 the cord or brain; they 

 are called efferent im- 

 pulses and the nerves 

 concerned in carrying 

 them, the efferent nerves. 

 FIG. 159. THREE NERVE CELLS After the dorsal and 



In two of them is shown the axon, or axis Ventral roots (afferent 

 cylinder, of the nerve connected with the cell. and efferen t) unite HltO 



a single trunk, the spinal nerve resulting is a mixture of both 

 kinds of fibres, though their functions remain distinct, In 

 the various figures in this chapter the direction of the arrows 

 indicates the direction of the impulses that pass through the 

 various nerves. The number of nerve fibres that thus enter 

 or find exit through the cord is very large. There are many 

 hundreds of thousands of them in the thirty-one pairs of spinal 

 nerves and by means of them every part of the body is brought 

 under the direct influence of the spinal cord and brain. 



Structure of Nerve Fibres. In the opening chapter the 

 minute cells which make up the bodv were described. These 



