490 HANDBOOK OF PHYSIOLOGY. 



abdominal muscles assist; their action being for the most part reflex and 

 involuntary. 



(/.) Centre for Movements of Lymphatic Hearts of Frog. Volkmann 

 has shown that the rhythmical movements of the anterior pair of lym- 

 phatic 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 oppo- 

 site 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 movementsof 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. 



(g.) The Centre for the Tone of Muscles. The influence of the spinal 

 cord on the sphincter ani and sphincter urethra has been already men- 

 tioned (see above). It maintains these muscles in permanent contrac- 

 tion. The condition of these sphincters, however, is not altogether ex- 

 ceptional. 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 sphincters 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 firmness 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. N 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 compact 

 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, modify or control them. The move- 

 ments or other phenomena of reflex action being as it were the function 

 of the ganglion cells to which an afferent impression is conveyed by the 

 posterior nerve-trunks in connection with them, and that the extent of 

 the movement depends upon the strength of the stimulus, the position 



