SYMPATHETIC NERVOUS SYSTEM. 257 



name. They supply, also, vasodilator fibers to rectum and anus 

 and motor fibers to the plain muscles of the colon descendens, 

 rectum, and anus. The preganglionic parts of these fibers end in 

 small sympathetic ganglia in the pelvic plexus or in the neighbor- 

 hood of the organs supplied. 



Normal Mode of Stimulation of the Autonomic Nerve Fibers. 

 In distinction from the nerve fibers innervating the skeletal 

 muscles practically the whole set of autonomic fibers is removed 

 from the control of the will. An apparent exception to this general 

 statement is found in the fact that the ciliary muscle of the eye is 

 seemingly under voluntary control. We must suppose that under 

 normal conditions the autonomic fibers are always excited 

 reflexly, and the course of the afferent fibers concerned in these 

 reflexes and the nature of the effective sensory stimulus in 

 each case are important in the consideration of each of the 

 physiological mechanisms involved. Most of these mechanisms, 

 as we shall find, work reflexly that is, without voluntary 

 initiation and, for the most part, unconsciously, for instance, 

 the movements of the intestines, the secretion of the digestive 

 glands, and the contraction and dilatation of the arteries. 

 The autonomic nerve-fibers control, therefore, the uncon- 

 scious co-ordinated actions, the so-called vegetative processes, 

 of the body. There is no apparent reason in the anatomical ar- 

 rangements why these fibers should be free from voluntary control. 

 Their distinguishing characteristic in comparison with the nerves 

 for the voluntary movements is the fact that they all terminate 

 first in sympathetic nerve cells; but this fact gives no explanation 

 of the absence of conscious control by the will. We are justified in 

 saying that nerve paths that pass through sympathetic nerve cells 

 cannot be excited voluntarily; but the immediate reason for this 

 fact is probably to be found in the ultimate point of origin of these 

 paths in the central nervous system. What we designate as vol- 

 untary motor paths arise in a definite region of the cortex, the 

 motor area in the frontal lobe. Our motor conceptions or ideas 

 can affect the efferent paths arising in this region, but not those, 

 apparently, which originate in other parts of the brain. 



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