252 



PHYSIOLOGY 



CHAP. 



density is therefore greater in the kathodic polar than in the 

 anodic peripolar region. 



As, therefore, the excitations and make contraction arise at 

 the kathode, and the excitation and break contraction at the 

 anode, Waller says that when a current strong enough to produce 

 contraction at break as well as at make is employed for unipolar 

 stimulation (anodal or kathodal) the kathodal closure contraction 

 is the strongest ; the kathodal opening contraction the weakest. 

 The anodai closure contraction is less strong than the kathodal, 

 and the anodal opening contraction is less weak than the kathodal. 



If instead of using strong currents, unipolar stimulation 

 commences with a weak current that is gradually strengthened, 

 the contractions (anodal and kathodal closure and opening) appear 

 in the following order : 



This order of contractions constitutes what Waller calls the 

 " law of contraction in man," which may be interpreted as follows : 



The fact that the kathodal make contraction is the first to 

 appear with weak currents, and is the strongest of all the reactions 

 with medium and strong currents, is due to its dependence upon 

 the katelectrotonus that arises in the polar regions, i.e. upon the 

 most effective form of stimulus in the most favourable region. 

 The appearance of the kathodal break contraction with strong 

 currents only, while it is the weakest of all the reactions, is due 

 to its dependence upon the disappearance of katelectrotonus in 

 the peripolar region, i.e. on a less effective form of stimulus in 

 the less favourable region. That the anodal make contraction 

 usually precedes the anodal break contraction can be explained by 

 the fact that the former depends on the appearance of katelectro- 

 tonus in the peripolar region, the latter on the disappearance 

 of katelectrotonus in the polar region. At other times, indeed 

 (Waller), this order may be inverted, and the anodal break con- 

 traction may precede the anodal make contraction. This anomaly 

 is only an apparent deviation from the law, and depends on the 

 relative density of current in the two regions, due to the nature 

 of the tissues that surround the nerve. 



According to Waller, the latent period of the break contraction 

 in man is constantly about 0'05 sec., i.e. it is extremely long in 



