THE NERVOUS SYSTEM 38 1 



cndocrinal and other glands, and the skin, so that there are few parts of 

 the body which autonomic fibers do not reach. Except possibly the 

 autonomic fibers connected with the ciUary muscle of the eye, autonomic 

 nerves, although markedly influenced by the emotions, are not under the 

 control of the will. 



Two kinds of autonomic nerves, sjnnpathetic and parasympathetic, 

 may be distinguished on the basis of their antagonistic action and their 

 different response to drugs. 



A sharp distinction between the sympathetic and parasympathetic 

 fibers cannot be drawn on the basis of function. The sympathetic 

 fibers are usually excitatory; the parasympathetic are usually inhibitory. 

 Most organs of the body have this double innervation and the action of 

 the two kinds of nerves is antagonistic. But some parasympathetic fibers, 

 as for example those in the vagus nerve, are excitatory. There are three 

 distinct groups of autonomic nerve fibers, a cranial group, a thoracico- 

 lumbar group, and a sacral or pelvic group. Of these, the cranial and 

 sacral elements are parasympathetic, and hence are grouped together as 

 the cranio-sacral division of the autonomic system. The thoracico-lumbar 

 division constitutes the sympathetic portion of the autonomic system. 



Autonomic nerves may be classified also, on the basis of their distribu- 

 tion, into somatic fibers which innervate the blood-vessels of the body- 

 wall and the smooth muscles of skin and sweat glands, and visceral 

 fibers which supply the glands and smooth muscles of the viscera. 



The somatic fibers act upon the hairs to stimulate their erection and 

 cause " goose flesh." They also serve the important function of regulating 

 temperature by influencing the tonus of the capillaries in the skin and 

 thus, by changing the rate of blood flow, altering the amount of secretion 

 of the sweat glands. (Fig. 117) 



The course followed by sympathetic and parasympathetic fibers within 

 the central nervous system is almost unknown. Evidence is not lacking 

 that stimulation of the cerebral cortex may be followed by reactions of 

 the viscera. 



Three kinds of autonomic fibers connect with sympathetic ganglia. 

 Preganglionic fibers are visceral efferent fibers which come from ganglion 

 cells located in the lateral column of the spinal cord and have their ter- 

 minations in sympathetic ganglia. Postganglionic fibers are also visceral 

 efferent and have their cell bodies within sympathetic ganglia and their 

 telodendria upon smooth muscles of the intestine and of the blood-vessels. 

 The preganglionic fibers are medullated and form the white rami com- 

 municantes which connect spinal nerves with sympathetic ganglia. The 

 postganglionic fibers are rarely medullated. They pass either to the 

 viscera by way of sympathetic nerves or to the body-wall and skin by way 



