50 THE HEART 



The skeletal muscle can also be studied under the combined 

 condition, namely, when " after-loaded." Here the condition of 

 the muscle is isometric while altering in tension to overcome the 

 load, and isotonic after this load has been overcome and the muscle 

 begins to shorten. A similar state of affairs prevails in the heart. 

 The ventricle contracts under isometric conditions upon its load 

 of blood until the semilunar valves open, thenceforward under 

 isotonic conditions whilst its fibres shorten and its load is dis- 

 charged. The heart therefore corresponds normally to an after- 

 loaded muscle. Frank showed by his studies that the laws which 

 govern the contraction of skeletal muscle under these conditions 

 apply equally to the heart. Thus, as in skeletal muscle, the iso- 

 metric curve culminates before the isotonic. He found also that 

 the maximum of the isometric curve of the ventricle increases with 

 increasing tension up to a certain point, and then decreases. This 

 means that the contraction of the ventricle, as is well known, in- 

 creases in force with the amount of its filling up to a certain point, 

 but after that it decreases and the ventricle dilates. Thus for 

 the frog's heart Frank found the following figures : 



Volume in com. in ventricle '18 -34 -47 '63 '84 '93 

 Tension mm. of mercury . 12 60 68 66 60 59 58 



During the isotonic curve the change of tension varies with 

 the load. Increasing load defers the beginning of the contraction, 

 shortens its duration, and diminishes the velocity of the move- 

 ment of the free end of the muscle. Similarly in the ventricle, 

 increased load that is, increased resistance to the outflow of blood 

 from the ventricle defers the beginning of the period of expulsion, 

 the opening of the semilunar valves, shortens the period until 

 their closure, and diminishes the rate of flow of blood through 

 the aortic orifice, and consequently lessens the systolic discharge. 

 As an example from experiments upon the frog's ventricle, if re- 

 sistance increases from 10 mm. to 40 mm. of mercury, the flow 

 decreases from "06 to '02 per second, and the period ot expulsion 

 diminishes from -56 second to '51 second, and the systolic dis- 

 charge from '33 cc. to '08 cc. This method of studying the 

 ventricle under both isometric and isotonic conditions gives valu- 

 able information in working out the effects of drugs upon the 

 heart muscle. 



So far we have spoken of the similarity between cardiac 



