REFLEX FUNCTION OF THE SPINAL CORD. 401 



sensitive fibres, being propagated to the spinal cord, excites 

 merely local spasms, spasms, namely, of those muscles, the 

 motor fibres of which arise from the same part of the spinal 

 cord as the sensitive fibres that are irritated. Of such a case 

 we have instances in the involuntary spasmodic contraction of 

 muscles in the immediate neighborhood of inflamed joints ; 

 and numerous other examples of a like kind might be quoted. 

 In other instances, in which we must assume that the cord 

 is morbidly more irritable, i. e., apt to issue more nervous 

 force than is proportionate to the stimulus applied to it, a 

 slight impression on a sensitive nerve produces extensive 

 reflex movements. This appears to be the condition in teta- 

 nus, in which a slight touch on the skin may throw the whole 

 body into convulsion. A similar state is induced by the in- 

 troduction of strychnia, and, in frogs, of opium, into the blood ; 

 and numerous experiments on frogs thus made tetanic, have 

 shown that the tetanus is wholly unconnected with the brain, 

 and depends on the state induced in the spinal cord. 



It may seem to have been implied that the spinal cord, as a 

 single nervous centre, reflects alike from all parts all the im- 

 pressions conducted to it. But it is more probable that it 

 should be regarded as a collection of nervous centres united in 

 a continuous column. This is made probable by the fact that 

 segments of the cord may act as distinct nervous centres, and 

 excite motions in the parts supplied with nerves given off from 

 them ; as well as by the analogy of certain cases in which the 

 muscular movements of single organs are under the control of 

 certain circumscribed portions of the cord. Thus Volkmann 

 has shown that the rhythmical movements of the anterior pair 

 of lymphatic hearts in the frog depend upon nervous influ- 

 ence derived from the portion of spinal cord corresponding to 

 the third vertebra, and those of the posterior pair on influence 

 supplied by the portion of cord opposite the eighth vertebra. 

 The movements of the heart continue, though the whole of the 

 cord, except the above portions, be destroyed ; but on the in- 

 stant of destroying either of these portions, though all the rest 

 of the cord be untouched, the movements of the correspond- 

 ing hearts cease. What appears to be thus proved in regard 

 to two portions of the cord, may be inferred to prevail in other 

 portions also ; and the inference is reconcilable with most of 

 the facts known concerning the physiology and comparative 

 anatomy of the cord. 



The influence of the spinal cord on the sphincter ani has 

 been already mentioned (p. 396). It maintains this muscle 



