TRANSMISSION OF IMPULSES. 447 



verse, passing into both the gray and white commissures, while others 

 lose themselves in the gray substance of the posterior and even of the 

 back part of the anterior horns. 



The connection of the anterior and posterior root fibres of the spinal 

 nerves with the cord is therefore not exactly the same ; but they both, 

 so far as known, first reach the gray matter, where a portion of them 

 terminate, or at least escape further observation, while the rest partly 

 become longitudinal on the same side, and partly cross over to the 

 opposite side of the spinal cord. 



Transmission of Motor and Sensitive Impulses in the Spinal Cord and 



Nerves. 



The experimental methods adopted for determining the functions of 

 different pnrts of the nervous system are twofold ; first, by applying an 

 artificial stimulus to the nerve or nervous tract, and observing the effect 

 which is produced ; secondly, by dividing or destroying it, and seeing 

 what nervous function is abolished in consequence of the operation. In 

 the peripheral parts of the nervous system, and in those generally which 

 serve as simple organs of transmission, both these methods yield defi- 

 nite and positive results. In the central parts, they are sometimes 

 complicated by the peculiar properties or the mutual reactions of the 

 gray and white substance. 



Motor and Sensitive Transmission in the Spinal Nerves and Nerve 

 Hoots. If the spinal canal of a living animal be opened, and a mechanical 

 or galvanic stimulus be applied to the anterior root alone of a spinal 

 nerve, the effect of this irritation is a convulsive movement of the part 

 to which the nerve is distributed. The muscular contraction is imme- 

 diate, involuntary, and instantaneous in duration ; and is repeated with 

 mechanical precision each time the stimulus is applied. It is usually 

 unaccompanied by any indication of sensibility, and is evidently a 

 direct result of the excitement of the nerve fibres of the anterior root. 

 This root is therefore said to be " excitable," because its irritation 

 excites a movement in the corresponding parts. 



Furthermore, if the anterior root of a spinal nerve, under the same 

 circumstances, be cut across, the remaining nervous connections being 

 left untouched, the result is an immediate and total paralysis of volun- 

 tary movement in the muscles to which that nerve is distributed. At 

 the same time, the power of sensibility is undiminished, and the animal 

 is still capable of feeling the contact of foreign bodies, or a galvanic cur- 

 rent applied to the skin, as before. If the anterior roots of a series of 

 spinal nerves be thus divided, as, for example, those of all the lumbar 

 and sacral nerves on one side, the above effect will be produced for an 

 entire corresponding region of the body, and the whole posterior limb 

 on that side will lose the power of voluntary motion while retaining its 

 sensibility. This is not clue to any loss of the physiological properties 

 of either the nerve or the muscles, since irritation of the nerve or nerve 

 root, outside the point of section, still produces muscular contraction as 



