508 THE XEfiVOUS SYSTEM. 



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 

 v 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. 501). 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 

 of the cord resembles its common reflex action in being in- 

 voluntary, although the will can act on the muscle to make 

 it contract more or to permit its dilatation, and in that the 

 constant action of the muscle is not felt, nor diminished in 

 sleep, nor productive of fatigue. But the act is different 

 from ordinary reflex acts in being nearly constant. In 

 this respect it resembles that condition of muscles which 

 has been called tone* or passive contraction ; in a state in 



* This kind of tone must be distinguished from that mere firmness 



