THE SPINAL CORD.— GENERAL. 467 



side be allowed to hang in the dilute acid, it will be withduawn. 

 4. If a small piece of blotting-paper dipped in the acid be 

 placed on the thigh, and the leg of that side gently held, the 

 other may be drawn up and remove the object. 



It may be noticed that in every case a certain interval of 

 time elapses before the result follows. Upon increasing the 

 strength of the acid very much this interval is shortened, and 

 the number of groups of muscles called into action is increased. 

 Again, the result is not the same in all respects when the nerve 

 of the leg is directly stimulated, as when the skin first receives 

 the impression. Section of the nerves of the parts abolishes 

 these effects ; so also does destruction of the spinal cord, or the 

 part of it with which the nerves of the localities stimulated are 

 connected ; and more exact experiments show that in the ab- 

 sence of the gray matter the section of the posterior or anterior 

 roots of the nerves also renders such manifestations as we have 

 been describing impossible. 



These experiments and others seem to show that an afferent 

 nerve, an efferent nerve, and one or more central cells are 

 necessary for a reflex action ; that the latter is only a perfectly 

 co-ordinated one when the skin (end-organs) and not the nerve- 

 trunks are stimulated ; that there is a latent period of stimula- 

 tion, suggesting a central " summation " of impulses necessary 

 for the effect ; that the reflex is not due to the mere passage of 

 impulses from an afferent to an efferent nerve through the 

 cord, but implies important processes in the central cells them- 

 selves. The latter is made further evident from the fact that 

 (1) strychnia greatly alters reflex action by shortening the 

 latent period and extending the range of muscular action, which, 

 it has been shown, is not due to changes in the nerves them- 

 selves. A very slight stimulus suffices in this instance to cause 

 the whole body of a decapitated frog to pass into a tetanic 

 spasm. We must suppose that the processes usually confined 

 to certain groups of central cells have in such a case involved 

 others, or that the " resistance " of the centers of the cord has 

 been diminished, so that many more cells are now involved; 

 hence many more muscles called into action. Normally there is 

 resistance to the passage of an impulse to the opposite side of the 

 cord, as is shown by the fact that when a slight stimulus is ap- 

 plied to the leg of one side the reflex is confined to this member. 



It is evident, then, that the reflex resulting is dependent on 

 (1) the location of the stimulus, (2) its intensity and duration. 



