CHAPTER XXVI 

 THE AUTONOMIC NERVE SYSTEM 



Introductory Statements. The motor tissues of the body may be 

 divided into two groups, the one, comprising the striated skeletal muscles, 

 the other, comprising the striated cardiac, and the non- striated vascular and 

 visceral muscles, and the epithelium of glands. The movements to which 

 they give rise are, in the first instance, those exhibited mainly by the head, 

 upper and lower extremities; and in the second instance, those exhibited 

 by the heart and blood-vessels; by the walls of the viscera, e.g., stomach, in- 

 testines, bile and urinary bladders, etc., and those exhibited by glands (the 

 secretion and discharge of fluids), e.g., salivary, gastric, perspiratory, mam- 

 mary, etc., as well as the specific agents, e.g., adrenalin, pituitrin, etc., dis- 

 charged into the blood. The activities which these two groups of motor 

 tissues exhibit are excited mainly by two ultimate and separate factors. 



The Skeletal Muscles. The skeletal muscles in the different regions of 

 the body are excited to action by nerve impulses, which come to them directly 

 from special groups of emissive or motor cells situated in the gray matter 

 beneath the aqueduct of Sylvius, beneath the floor of the fourth ventricle, 

 and in the ventral cornua of the gray matter of the spinal cord. The nerve 

 impulses, discharged by these cells, are transmitted directly through but one 

 axon to the muscle. The nerve-cells in these different regions, though 

 irritable, do not possess spontaneity of action, but require for the manifesta- 

 tion of their activity the arrival and the stimulating influence of nerve im- 

 pulses. These may come (i) from peripheral regions of the body through 

 afferent spinal nerves in consequence of the action of external agents, in 

 which case the resulting movement is termed a reflex movement; or (2) from 

 the cerebrum through descending nerve-fibers in consequence of an act of 

 volition, in which case the resulting movement is termed a volitional or 

 voluntary movement. In the performance of daily work the skeletal muscle 

 activities, though frequently caused by reflected nerve impulses are, in the 

 vast majority of instances, caused by nerve impulses of cerebral (volitional) 

 origin. From moment to moment these muscles are excited to action, guided 

 and controlled for the most part by this dominating and in some respect 

 arbitrary factor. By the constant exercise of the will, these muscles are 

 compelled to act constantly, and for this reason it is said that they do not 

 possess independent activity, except in a minor degree. 



The Striated Cardiac, the Non-striated Vascular and Visceral 

 Muscles and the Epithelium of Glands. The striated cardiac, the vas- 

 cular and visceral muscles, and the epithelium of glands in all regions of the 

 body, if not primarily excited to action, are at least controlled and regulated 

 in their activities, by nerve impulses that come to them indirectly from special 

 groups of emissive or motor nerve-cells situated in (i) a small region at the 

 extreme upper part of the gray matter beneath the aqueduct of Sylvius; (2) 

 in special regions in the gray matter beneath the floor of the fourth ventricle; 

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