634 THE AUTONOMIC NERVOUS SYSTEM 



At the hand of these details, we are now in a position to explain 

 why typically autonomic functions may also be had in regions which 

 on casual observation seem to be innervated exclusively by a cerebro- 

 spinal nerve. Thus, we observe that vasomotor and secretomotor 

 actions are not restricted to the viscera, but are also enacted in the 

 integument and deeper structures of the trunk, arms and legs. It 

 must be inferred, therefore, that the spinal nerves innervating these 

 parts, derive their supply of sympathetic fibers by way of the gray 

 rami (IV). In this way, their original power of regulating the activity 

 of the striated musculature is augmented by the control of the smooth 

 muscle and glandular tissue. To illustrate, the sciatic nerve contains 

 first of all a certain number of fibers for the skeletal muscles of the 

 leg, secondly, fibers for the smooth muscle of the blood-vessels (vaso- 

 motor) and skin (pilomotor) and thirdly, fibers for the sweat glands 

 (secretomotor) of this part. The former pursue a straight course 

 from the spinal cord to their peripheral effectors (I), while the latter 

 are first diverted into the sympathetic ganglia, by way of the white 

 rami (II), whence they are again directed into this spinal nerve by 

 way of the gray rami (IV). For this reason, they are frequently desig- 

 nated as recurrent fibers. It seems quite probable that a similar 

 arrangement exists at the points of union between the cranial nerves 

 and the sympathetic, or more correctly speaking, the parasympathetic 

 system. 



The peculiar mannet of distribution of these fibers is well illustrated 

 by that of the pilomotors. 1 Using the cat as an example, it is found 

 that the latter leave the spinal gray matter by way of the anterior 

 roots of the fourth thoracic to third lumbar nerve. They enter the 

 sympathetic system through the white rami, where they arborize in the 

 ganglia of this chain to form connections with neighboring ganglia 

 above and below their point of entrance. Each ganglion in turn 

 remits a certain number of secondary fibers which again reach the 

 corresponding spinal nerve by way of the neighboring gray ramus. 

 Prom here they are distributed to the smooth muscle cells of the skin 

 of that particular region. The fact that the sympathetic ganglia 

 permit of a spreading of the primary impulse may be proved by the 

 stimulation of the neighboring white and gray rami. For example, 

 while the excitation of a certain gray ramus will yield pilomotor effects 

 only in that segment of the body to which the corresponding mixed 

 nerve is distributed, the stimulation of the neighboring white ramus 

 most generally evokes these effects in the areas of the three or four 

 adjoining spinal nerves. Obviously, this result can -only be obtained 

 if the primary impulse is relayed to neighboring efferent paths, and 

 naturally, there is every reason to believe that this spreading is not 



1 While the production of "goose flesh " and the erection of the hairs are usually 

 classified as involuntary phenomena, cases have been placed on record which 

 show that individuals may acquire an accurate voluntary control over these 

 otherwise purely sympathetic reactions. 



