THE NERVOUS IMPULSE 97 



Other Autonomic Nerves. Langley has introduced the term autonomic 

 to include "the contractile cells, unstriated muscle, cardiac muscle, and 

 gland cells of the body, together with the nerve cells and fibers in con- 

 nection with them. The autonomic nervous system consists of the sym- 

 pathetic system, which we have already in part considered, of the cranial 

 autonomic system, the sacral autonomic system, and the enteric system." 

 The nerves supplying the sorts of tissue first-named above not supplied 

 by the sympathetic come from the other autonomic nerves, cranial, 

 sacral, and enteric. In the most general terms the cranial autonomic 

 nerves (the third, seventh, ninth, tenth, and eleventh) supply certain 

 parts and functions at the upper end of the alimentary canal. The 

 sacral autonomic system (which are all fibers connected with the pelvic 

 nerve, "nervus erigens") serves various parts near the lower end of the 

 same canal rectal muscles, generative muscles, vasomotor muscles, etc. 

 The enteric system consists of the plexuses of Auerbach and of Meissner. 

 These are unique neural nets extending from the middle of the esophagus 

 to the anus. Whether these two systems of nerve-cells and fibers are 

 connected with the cord and brain by sympathetic or by cranial and 

 sacral autonomic fibers, is as yet not known. They are so unlike the 

 other nerves of the sympathetic that they may be properly classed 

 separately. 



This division of what used to be known as the " sympathetic system" 

 into the sympathetic, cranial, sacral, and enteric systems is a step in the 

 important direction of unravelling the set of nerve-fibers whose general 

 functions we have just suggested. The danger is rather of too little 

 analysis and classification than of too much, for scarcely yet is the 

 complexity of the nerve-fibrils necessary to the actually observed com- 

 plexity of function adequately realized. 



THE NERVOUS IMPULSE. 



Despite the large amount of work put upon it, the real nature of 

 the nervous impulse is today as uncertain as ever it was. There have 

 been many speculations as to the means by which the nervous influence is 

 transferred along the nerve. Some of these now appear absurd indeed 

 to us. For example, it was at one time supposed that the nerve-fiber 

 was in effect a string which was in some way pulled on by the brain, 

 much as a boy pulls a jumping-jack. Another supposition was that the 

 nerve was a tube and that the impulse passed literally in a stream 

 through it. Some thought that the movement along the nerve was a 

 mechanical vibration, as others still think that it may be a vibration of 

 a molecular sort. More recent suppositions have been that the impulse 

 is of an electrical nature, such as runs along a telegraph wire. A still 

 more recent supposition is that it is a progressive coagulation of a 

 colloidal material of the nerve tissue brought about by ionic influences. 

 Finally, we may note that it is commonly thought that the impulse is a 



