102 EXPEEIMENTAL PHYSIOLOGY 



Place it upon a clean glass plate. The process of cutting will be found 

 to exert an influence upon the rhythm. Usually it is inhibited for the 

 space of a few beats and then recovers and beats very rapidly for a 

 short time, but quickly settles down to a regular rate, which, as a rule, 

 is slower than that in situ, though the reverse may at times occur. 

 When it has settled down count and make a note of this rate per 

 minute. Cut away the sinus at its junction with the auricles. The 

 effect upon the auricles and ventricle is that they give a series of rapid 

 beats and then come to a standstill completely relaxed. The effect 

 upon the sinus is usually found to be inhibitory. It stops beating at 

 once, though in a few cases, if care be taken to injure it as little as 

 possible, it may continue its rhythm unaltered. After a short period 

 both parts recommence to beat, but with quite a different rhythm. 

 Count the rate per minute. The auricles and ventricle beat at a rather 

 less rapid rate than the whole heart. The sinus being more injured 

 takes longer to recover and beats at a slower rate. If the course of the 

 experiment be modified, in that instead of removing the sinus with the 

 rest of the heart the incision be made between sinus and auricles, so 

 as to leave the sinus in situ, it is usually found that the sinus beat is 

 from the first but little affected, and frequently quite unaltered. Next 

 remove the auricles from the ventricle, cutting on the auricular side 

 of the auric ulo- ventricular groove so that a small edge of auricle 

 is still attached to the ventricle. The process of cutting again inhibits 

 both auricle and ventricle. After a time both recommence to beat the 

 auricle earlier than the ventricle. If the amount of auricle still 

 attached to the ventricle be very small the ventricle may not of itself 

 recommence beating. If it do not, it may be taught to beat by 

 stimulating it rhythmically with mechanical (e.g. a prick of a needle), 

 or with electrical stimuli repeated at regular intervals. In many cases 

 it will then be found that the ventricle starts off beating quickly and 

 gradually slows down until it ceases altogether. The rhythm may 

 then be again started by rhythmic stimuli. If the ventricle recover 

 its rhythm cut through it just below the auriculo-ventricular groove : 

 the ventricle is brought to a standstill and does not regain an 

 automatic beat if left to itself. With each mechanical stimulus a 

 contraction follows, and if the stimuli be repeated rhythmically for 

 a time, say one per second, the ventricle may regain an automatic 

 rhythm, though this does not often occur with a frog's heart. If it 

 should, count the rate. 



The results of this experiment should be collected together in a 

 tabular form somewhat after the plan adopted in the following experi- 

 ment, which is slightly modified from the preceding instructions : — 



