CHAPTER XCVI 



THE AUTONOMIC NERVOUS SYSTEM, OR THE EFFERENT PATH- 

 WAY TO SMOOTH MUSCLES AND GLANDS 



The development of complex masses of association neurons in the cen- 

 tral nervous system of the higher animals is associated with the acquisi- 

 tion of more and more specialized motor mechanisms and of a diversity in 

 the responses which these muscular structures can make. The organs con- 

 cerned with nutrition have retained many of their primitive characters 

 such as walls of smooth muscle and innervation by nonmedullated neu- 

 rons, arranged in some cases, at least, as a nerve net. The nervous 

 mechanism for the control of skeletal muscle has developed, consequently 

 as an adjunct to the visceral system, and has reached such a dominating 

 position that the latter has been rather neglected in the hands of neu- 

 rologists. There can be little doubt, however, as the interesting book 

 of Pottenger 18 suggests, that the study of visceral neurology will con- 

 tribute greatly to elucidating the symptoms of disease. Because the 

 neurons of the autonomic nervous system are organized somewhat differ- 

 ently than are the efferent paths to skeletal muscle, and innervate organs 

 of a different function, the mistake should not be made of thinking that 

 it functions independently of the central nervous system. There is no 

 evidence that afferent impulses play upon these neurons except after 

 passage into the central nervous system. We have seen that afferent 

 impulses from the viscera may give rise to sensory effects in the central 

 nervous system (page 873). Similarly reflex change in the viscera may 

 be set up through impulses having an efferent path in the autonomic 

 nervous system, but originating from stimuli acting upon the surface 

 of the body. The pupillary reflex elicited by pinching the neck is a case 

 in point as is the psychic secretion of gastric juice, the voluntary 

 emptying of the bladder, and the syndrome of gastrointestinal and 

 circulatory changes which accompany great emotion. The autonomic 

 nervous system comprises the group of neurons which carry impulses 

 to the smooth muscles and glands of the body. It owes its interest and 

 importance to the functions to which these tissues contribute (nutrition 

 and reproduction), rather than to any supposed autonomy it may possess. 



The Organization of Efferent Nerves to the Viscera 



The reflexes in which the autonomic neurons take part are conducted 

 over reflex arcs. While the afferent side of such arcs may arise from 



893 



