LII.] THE frog's heart. 26 1 



(c.) Place the heart on a microscopical slide and note that during 

 diastole it is soft and flaccid, and adjusts itself to any surface it may 

 rest on. During systole, i.e., when it contracts, its apex is raised 

 and erected. 



6. Heat and Cold on the Excised Heart. 



(a.) Place the watch-glass containing the beating heart on the 

 palm of the hand, and the heart beats faster ; or place it on a beaker 

 containing warm water, which must not be above 40° C. Note 

 that, as the temperature of the heart rises, it beats faster-- there are 

 more beats per minute — therefore each single beat is faster. 



{h.) Place the watch-glass and heart over a beaker containing iced 

 water, the number of beats diminishes, each beat being executed 

 more slowly and sluggishly. 



7. Section of the Heart. 



(a.) Expose the heart, divide the pericardium, and hgature the 

 frsenum, and by means of it gently raise the heart. With scissors 

 excise the whole heart, including the sinus venosus. The heart still 

 beats. 



{h.) Cut off the sinus ; it continues to beat. The rest of the 

 heart ceases to beat for a time, but by-and-by it commences to beat 

 rhythmically. 



(c. ) Sever the auricles from the ventricle ; the ventricle ceases 

 to beat. The ventricle, however, has not lost the power of beating 

 rhythmically. To prove this, stimulate it mechanically, e.g.^ by- 

 pricking it with a needle. After an appreciable latent period, it 

 executes one — generally several — beats, and then becomes quiescent. 

 Stimulate with a single induction shock, this also causes it to dis- 

 charge one or more beats. 



{(J.) Cut ofi' the apex of the ventricle ; it remains quiescent ; but 

 if it be stimulated, either mechanically or electrically, it makes a 

 single beat — not a series, as in the case of {(..). 



(e.) Divide the ventricle of another heart below the auriculo- 

 vcntricular groove. The auricles, with the upper part of the ventricle 

 attached, continue to beat, while the lower two-thirds no longer 

 beats spontaneously. If it be pricked with a needle, however, it 

 contracts as often as it is stimulated mechanically. It responds by a 

 single contraction to a single stimulus, but a single stimulus does 

 not excite a series of rhythmical contractions. 



(J.) With scissors divide longitudinally the auricles with the 

 attached portion of the ventricle. Each half contracts spontaneously, 

 although the rhythm may not be the same in both. 



{g.) Instead of cutting, one may use a ligature, or the heart apex_ may be 

 separated by Bernstein's method, viz., compress the heart above its apex 



