484 ANIMAL PHYSIOLOGY, 
general and minute anatomy of the spinal cord, we shall pro- 
ceed to discuss its functions. 
THE REFLEX FUNCTIONS OF THE SPINAL CoRD. 
The following experimental observations may readily be 
made by the student himself: Let a decapitated frog be sus- 
pended freely (from the lower jaw). It bangs motionless and 
limp at first, but when it recovers from the shock (abolition of 
function) to the spinal cord produced by the operation, it may 
be shown that this organ is functional: 1. When a piece of 
bibulous paper dipped in dilute acid is placed upon the thigh, 
the leg is drawn up and wipes away the offending body. 2. If 
the paper be placed on the anus, both legs may be drawn up, 
either successively or simultaneously. 3. If the leg of one 
side be allowed to hang in the dilute acid, it will be withdrawn. 
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 absence of the gray matter the section of the pos- 
-terior or anterior roots of the nerves also renders such mani- 
festations 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 
stimulation, 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 cen- 
tral cells themselves. The latter is made further evident from 
the fact that (1) strychnia greatly alters reflex action by short- 
