THE AUTONOMIC NERVE SYSTEM 643 



(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- 

 Jandionic, 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 

 asents 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 ir 

 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 

 maiority of instanced stimulated to activity by nerve impulses transmitted 

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

 ing phases of affective or emotional psychic activity) this systerr 

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

 or autonomous in its activity, and, therefore, has been des' 

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

 akn been designated the autonomic tissues. 



In a cons deration of this subject it must be borne in ^md that there are 



grees, by peripheral and cerebral activities. 



i It fa onty necessary to recall the ^ 

 vessels and sweat-glands of the necl ; d ace 1 = oserva ^ o Cannon of the effects 



- 



c"teo and intesunal move - 



merits and the secretion of the adrenal glands. 



