REFLEX FUNCTION OF THE SPINAL CORD. 509 



the whole body into convulsion. A similar state is induced 

 by the introduction 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 uncon- 

 nected 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 

 impressions 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 ner- 

 vous 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 por- 

 tions of the cord. Thus Volkmann has shown that the 

 rhythmical movements of the anterior pair of lymphatic 

 hearts in the frog depend upon nervous influence derived 

 from the portion of spinal cord corresponding to the third 

 vertebra, and those of the posterior pair on influence sup- 

 plied 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 instant of destroying either of these portions, though 

 all the rest of the cord be untouched, the movements of 

 the corresponding hearts cease. What appears to be thus 

 proved in regard to two portions of the cord, may be in- 

 ferred 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. 502). It maintains this muscle 

 in permanent contraction, so that, except in the act of defeca- 

 tion, the orifice of the anus is always closed. This influence 



