NERVE. 55 



on a motor nerve the latent period for contraction of the muscle 

 is greater at make when the kathode is in the far position, than 

 when it is in the near position. Likewise at the break the latent 

 period is greater when the anode is in the far position than when 

 in the near position. Therefore the galvanic make stimulus is 

 kathodal and the break stimulus anodal. 



Electrotonus, Kathodal and Anodal Contractions in the 



Heart. Because of the regularly occurring contractions the heart 



is an admirable muscle for the study of electrotonic changes as 



well as the effect of kathodal and anodal stimulation. 



*Experiment 25. Destroy the brain of a frog and expose the heart 



with as little hemorrhage as possible. Two boot electrodes are 



prepared by attaching a tiny strip of cotton wool to each at 



the toe. The cotton is soaked with isotonic NaCl solution 



and serves as a contact. The kathode is made to touch the 



frog's mouth while the anode touches the ventricle. Now if the 



current is closed during contraction of the ventricle, it will be 



seen that at the moment of closing the latter fails to contract 



in the region of the anode. This is due to the anelectrotonus 



set up around the anode. Of course only a limited area will be 



affected, so that very close observation will be necessary.* 



Now if the current is broken during diastole the ventricle 

 will contract slightly in the neighbourhood of the anode. 

 This is the anodal opening contraction. 



The condition produced in muscle by the kathode can be 

 studied by changing the electrodes so that the kathode touches 

 the ventricle and the anode the mouth. At the closing of the 

 circuit the ventricle contracts in the region of the kathode, if 

 it is in diastole. This is called the kathodal closing contrac- 

 tion. If the current is passing during systole there may be a 

 small area of heightened contraction around the kathode. 

 This is due to the katelectrotonus. 



The response at make and break depends upon the strength 

 of current and upon whether the kathode or the anode is next to 

 the muscle. 



With the KATHODE NEAR THE MUSCLE a contraction is obtained 



*The local contraction or failure of contraction is revealed by a change in 

 color due to variations in the blood content of the heart tissue as the affected part. 



