476 COMPARATIVE PHYSIOLOGY. 



these phenomena certainly teach the dependence of one part 

 upon another in the normal animal, and should make one cau- 

 tious in drawing conclusions from any kind of experiment, in 

 regard to the normal functions. As we have often urged in 

 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 diflferent, 

 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 in a 

 somewhat stretched tondition, 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- 



