CHAP. XI.] MECPIANISM OF THE ACTION OF THE CORD. 323 



that each afferent nerve has its proper efferent one, the former being 

 excitor, the latter motor. The aggregate of these fibres together with 

 the gray matter constitutes the true spinal cord of Dr. Marshall 

 Hall, which is not limited to the spinal canal, but passes up into 

 the cranium as far as the crura cerebri. These fibres are quite 

 independent of those of sensation and volition, and of the sensorium 

 commune. Although bound up with sensitive and motor fibres, 

 they are not affected by them, and they maintain their separate 

 course in the nerves as well as in the centres. Such is the hypo- 

 thesis of an excito-motory system of nerves, and of a true spinal cord, 

 the centre of all physical nervous actions, which has been proposed 

 and most ably advocated by Dr. Marshall Hall. 



2. The fibres of sensation and volition proceed to and from some 

 part or parts of the intra-cranial mass. Those which are distributed 

 to the trunk pass along the spinal cord, separating from it with the 

 various roots of the nerves, and in their course within the spine 

 they mingle more or less with the gray matter. There are no other 

 fibres but these (save the commissural) , and they are sufficient 

 to manifest the physical as well as the mental acts. Nerves of sen- 

 sation are capable of exciting nerves of motion which are in their 

 vicinity ; and they may produce this effect even when the spinal 

 cord has been severed from the brain, for their relation to the gray 

 matter of the cord is such that their state of excitement is readily 

 conveyed to it. This explanation tallies with the views of 

 Whytt, Prochaska, and the other physiologists who had recognized 

 the existence of a class of actions produced by the influence of 

 sensitive upon motor nerves. 



3. According to a third hypothesis, it is assumed that all the 

 spinal and encephalic nerves, of whatever function, are implanted 

 in the gray matter of the segments of the cerebro- spinal centre with 

 which they are severally connected, and do not pass beyond them. 

 The segments are connected with each other through the continuity 

 of the gray matter from one to another, and through the medium 

 of commissural fibres which pass between them. Through these 

 means, motor or sensitive impulses are propagated from segment to 

 segment ; and a stimulus conveyed to any segment from the peri- 

 phery may either simultaneously affect the brain and cause a sen- 

 sation, or be reflected upon the motor nerves of that segment and 

 stimulate their muscles to contract. 



The first hypothesis, which assumes the existence of a distinct 

 series of incident and reflex nerves for the physical nervous action, 

 offers a very beautiful explanation of those cases in which, while 



Y2 



