104 HAND-BOOK OF PHYSIOLOGY. 



IKIS 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 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 

 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 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. 



Tone of Muscles. The influence of the spinal cord on the sphinc- 

 ter ani (centre for defecation) has been already mentioned (see above). 

 It maintains this muscle in permanent contraction, so that, except in 

 the act of defaecation, the orifice of the anus is always closed. This 

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

 tary, although the will can act on the muscle to make it contract more, 

 or may inhibit the action of the ano-spinal centre so as to permit its dila- 

 tation. The condition of the sphincter ani, however, is not altogether 

 exceptional. It is the same in kind, though it exceeds in degree that 

 condition of muscles which has been called tone, or passive contraction; 

 a state in which they always when not active appear to be during health, 

 and in which, though called inactive, they are in slight contraction, and 

 certainly are not relaxed, as they are long after death, or when the spinal 

 cord is destroyed. This tone of all the muscles of the trunk and limbs 

 depends on the spinal cord, as the contraction of the sphincter ani does. 

 If an animal be killed by injury or removal of the brain the tone of the 

 muscles may be felt and the limbs feel firm as during sleep; but if the 

 spinal cord be destroyed, the sphincter ani relaxes, and all the muscles 

 feel loose, and flabby, and atonic, and remain so till rigor mortis com- 

 mences. This kind of tone must be distinguished from that mere firm- 

 ness and tension which it is customary to ascribe, under the name of tone, 

 to all tissues that feel robust and not flabby, as well as to muscles. The 

 tone peculiar to muscles has in it a degree of vital contraction: that of 

 other tissues is only due to their being well nourished, and therefore com- 

 pact and tense. 



All the foregoing examples illustrate the fact that the spinal cord is a 

 collection of reflex centres, upon which the higher centres act by sending 

 down impulses to set in motion, to modify or to control them; the 

 movements or other phenomena of reflection being as it were the function 

 of the ganglion cells to set in action, after an afferent impression has been 

 conveyed to them by the posterior nerve-trunks in connection with them. 

 The extent of the resulting movement depends upon the strength of the 



