THE HEART 49 



degree of excitability to artificial stimuli ; later the automatism, 

 especially that of the ventricle, decreases, while its excitability not- 

 ably increases. Similarly the auricle possesses at this time a greater 

 power of automatism than the ventricle, but the ventricle is the 

 more excitable of the two chambers (Fano). The power of con- 

 tractility also varies in different parts of the heart, being greatest 

 in the ventricle, the part of the cardiac tube developed for pro- 

 pulsive work. But in dying this power is preserved longer in the 

 auricle than in the ventricle, the parts of the right auricle around 

 the superior vena cava and round the coronary sinus being the 

 last to lose this property. 



The muscle curve obtained as the result of the application of 

 a stimulus corresponds to that of an ordinary muscle. There is 

 the period of delay, the period of contraction, and the period of 

 relaxation. In point of time it approaches more nearly to that 

 of smooth muscle ; the latent period is well marked, and the period 

 of contraction is considerably slower than that of the sartorius 

 or of the gastrocnemius of the same animal. As with smooth 

 muscle, the heart appears to be more easily stimulated by gradual 

 than by momentary shocks. 



As with skeletal muscle, the contraction* of the heart can 

 be recorded under two conditions, namely, the isotonic and the 

 isometric. This has been done more especially by 0. Frank. The 

 conditions in the heart, however, are not directly comparable 

 with those in the. skeletal muscle. In the latter, under isotonic 

 conditions the actual shortening of the muscle under a constant 

 weight is measured ; in the heart, owing to the anatomical arrange- 

 ment of its fibres, it is impossible to do this. The isotonic curve 

 is therefore obtained by measuring the alteration in the size of 

 the cavity of the ventricle which occurs as the result of this shorten- 

 ing of the fibres. This is done by introducing a sound into the 

 ventricle and connecting it with a tambour. The isometric curve 

 of skeletal muscle is obtained by preventing the muscle from 

 shortening while the change in its tension is measured. The 

 allied condition is brought about in the heart by causing it to 

 contract against an insuperable obstacle, such as a tap introduced 

 into the circuit. The tension of the ventricle wall alters under 

 these conditions, but the length of the fibres remains the same. 

 This alteration in tension is measured by a manometer inserted 

 between the ventricle and the insuperable obstacle. 



D 



