494 ANIMAL PHYSIOLOGY. 
the foregoing chapters, what a part may under certain circum- 
stances manifest, and what its behavior may be as usually - 
placed in its proper relations in the body, are entirely different, 
or at least may be. When one leg is laid over the other and a 
sharp blow struck upon the patella tendon, the leg is jerked up 
in obedience to muscular contraction. It is not a little difficult 
to determine whether this result is due to direct stimulation of 
the muscle or to reflex action, the first link in the chain of 
events necessary to call it forth originating in the tendon; 
hence the term tendon-reflex. But at present it is safer to 
speak of it as the “ knee-jerk,” or the “ tendon-phenomenon.” 
It disappears, however, when the spinal cord is destroyed or is 
diseased, as in locomotor ataxia, or when the nerves of the 
muscles or the posterior nerve-roots are divided, showing that 
the integrity of the center, the nerves, and the muscles are all 
essential. There are normally many such phenomena (reflexes) 
besides the “ knee-jerk.” 
Another question very difficult to decide is that relating to 
the usual condition of the muscles of the living animal. It is 
generally admitted that the muscles of the body are all ina 
somewhat stretched condition, but it is not so clear whether 
the skeletal muscles are under a constant tonic influence like 
those of the blood-vessels. It is certain that, when the nerves 
going to a set of muscles are cut, when even the posterior roots 
of the nerves related to the part involved are divided or the 
spinal cord destroyed, there is an unusual flaccidity of the 
limb involved. But the natural condition may be, it has been 
suggested, the result of reflex action. The subject is probably 
more complex than it has hitherto been considered. 
The facts of such a case—those of the tendon-phenomenon 
and similar ones—would be better understood if the spinal 
cord, the nerves, and the muscles associated with them, were 
regarded as parts of a whole so connected in their functions 
that severance of any one of them leads to disorder of the rest. 
That the cells of the cord are constantly exercising an influence 
through the nerves on the muscles, while they in turn do not 
lead an independent existence, but are as constantly influenced 
by afferent impulses, and that one of the results is the condi- 
tion of the muscles referred to, is, we are convinced, the case. 
To say that it is either entirely automatic or purely reflex, or 
that the whole of the facts would be covered even by any com- 
bination of these two processes, would probably be unjustifi- 
able. The influence of the centers over the metabolism of parts 
