3OO THE NERVOUS SYSTEM 



dependent for power upon their fibers from the cerebro- 

 spinal system, while others seem capable of acting indepen- 

 dently, at least for a time. 



Functions. Little is known of the functions of the sym- 

 pathetic except as regards efferent fibers. They are dis- 

 tributed in general to the non-striped musculature of the cir- 

 culatory apparatus and of the viscera, to secreting glands 

 and to the heart. The heart furnishes the only example of a 

 direct sympathetic supply to striated muscle. The sympa- 

 thetic has a very definite effect upon secretion, nutrition and 

 the local production of heat. Section of the sympathetic 

 fibers going to any part causes hyperemia, an increased 

 amount of secretion (sweat, e. g.), and a rise of temperature 

 in that part. The last two conditions are caused by the first, 

 and it in turn is due to a paralysis of the muscular coat of the 

 vessels, allowing an abrogation of their usual tonic condition 

 and, consequently, dilatation and an increased amount of 

 blood with exaggerated nutritive activity. This statement 

 confronts us with the question of vaso-motor action. 



Vaso-motor Phenomena. By vaso-motor nerves is meant 

 those fibers which convey to the muscular coat of the vessel 

 walls impulses causing them to contract and decrease the 

 caliber, or to relax and increase it. Those causing contrac- 

 tion are called vaso-constrictors ; those causing relaxation 

 vaso-dilators. It is mainly through the operation of vaso- 

 motor nerves that the sympathetic system influences nutri- 

 tion in a particular part, though all vaso-motor fibers are not 

 confined to the sympathetic cords. However, it is not 

 through the operation of the vaso-motor nerves alone that 

 the sympathetic lays claim to be the "system of nutrition," 

 for all the parts to which its other fibers are distributed con- 

 tribute also very materially to nutrition, though perhaps in 

 not so direct a manner as do the muscular coats of the ar- 

 teries. While intestinal peristalsis, the secretion of many 

 glands, as, for example, the production of glycogen, bile, etc., 

 cannot be shown to be absolutely dependent on sympathetic 



