236 PHYSIOLOGY OF CENTRAL NERVOUS SYSTEM. 



but probably they comprise the small and, for the most part, un- 

 named local ganglia found in or near the organs innervated. 



General Course of the Autonomic Fibers Arising from the 

 Sacral Cord. The autonomic fibers of this region emerge from 

 the cord in the anterior roots of the sacral nerves, second to 

 fourth. The branches from these roots unite to form the so-called 

 nervus erigens (pelvic nerve), which loses itself in the pelvic plexus 

 without making connections with the sympathetic chain of ganglia. 

 The pelvic plexus is formed in part also from the hypogastric nerve 

 arising from the inferior mesenteric ganglion. Through this latter 

 path autonomic fibers from the upper lumbar region enter the 

 plexus (Fig. 105). The autonomic fibers of the nervus erigens 

 supply vasodilator fibers to the external genital organs, and in 

 the male constitute the physiological mechanism for erection; 

 whence the name. They supply, also, vasodilator fibers to rectum 

 and anus and motor fibers to the plain muscles of the colon de- 

 scendens, rectum, and anus. The preganglionic part of these fibers 

 ends in small sympathetic ganglia in the pelvic plexus or in the 

 neighborhood 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 they 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 con- 

 sideration 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, uncon- 

 sciously, for instance, the movements of the intestines, the secre- 

 tion of the digestive glands, and the contraction and dilatation of 

 the arteries. The autonomic nerve fibers control, therefore, the 

 unconscious 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- 



