6 5 g TEXT-BOOK OF PHYSIOLOGY 



(3) in the ventral horns of the gray matter of the spinal cord between and 

 including the levels of origin of the second thoracic to the second or third 

 lumbar nerves; and (4) in the gray matter of the lower portion of the cord 

 between and including the levels of origin of the second to the fourth sacral 

 nerves. The nerve impulses discharged by these cells are transmitted indi- 

 rectly, not through one, but through two successively arranged neurons. The 

 first, a fine white medullated fiber, emerges from the medulla or spinal cord 

 in association with the large motor root fibers passing to the skeletal muscles, 

 and after a variable distance leaves these fibers to arborize around and to 

 become physiologically related to a sympathetic ganglion; the second, a fine, 

 dark non-medullated fiber, emerges from one of the cells of the ganglion, 

 which, after pursuing a longer or shorter course, branches and becomes his- 

 tologically and physiologically related to non-striated muscle-fibers and 

 epithelium. The first neuron is termed pre-ganglionic, the second post- 

 ganglianic, or in accordance with long-established usage, sympathetic. 



The nerve-cells in the regions which give origin to the pre-ganglionic 

 neurons, though irritable, do not possess spontaneity of action, but require 

 for the manifestation of their activities, the arrival and stimulating influence 

 of nerve impulses. These may likewise come (i) from peripheral regions 

 of the body through afferent 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 affective or emotional psychic states, in which case the resulting movement 

 or modification of movement, is termed an affective or an emotional move- 

 ment. In the performance of the functions of vascular and visceral organs 

 and glands, the non-striated muscles and epithelium are in the vast majority 

 of instances caused by reflected nerve impulses, though frequently modified 

 by nerve impulses of cerebral (affective or emotional) origin. 1 



From the fact that the nerve-centers of the pre-ganglionic nerve-fibers in 

 the cranio-bulbar and spinal-cord regions are removed from and not subject 

 to volitional control; and from the further fact that they are in the vast 

 majority of instances stimulated to activity by nerve impulses transmitted 

 from peripheral regions (though modified from time to time by the ever vary- 

 ing phases of affective or emotional psychic activity) this system of nerves is 

 regarded as in a sense independent, i.e., of volitional control, self -regulative 

 or autonomous in its activity, and, therefore, has been designated the auto- 

 nomic nerve system (Langley). The tissues to which it is distributed have 

 also been designated the autonomic tissues. 



In a consideration of this subject it must be borne in mind that there are 

 not two nerve systems, but only one, which, however, may be subdivided 

 into two portions, one of which in its peripheral distribution is associated 

 directly with skeletal muscles only; the other of which is associated indirectly, 

 through the intervention of a ganglion, with non-striated muscle and gland 

 epithelium. The central origin of these two portions is essentially the same, 

 and the central cells are influenced in the same way, though in different de- 

 grees, by peripheral and cerebral activities. 



1 It is only necessary to recall the well-known effects of psychic states on the heart, on the blood- 

 vessels and sweat-glands of the neck and face; the observations of Pavlov on the effects of agreeable 

 psychic states on the flow of saliva and gastric juice; the observations of Cannon of the effects 

 of psychic states of an opposite character of fear, anger, etc., on the gastric and intestinal move- 

 ments and the secretion of the adrenal glands. 



