442 



THE PROPERTIES OF STRIPED MUSCLE. 



orifice, and consequently to diminish the systolic discharge. Stated 

 numerically, the result of such experiments on the frog's ventricle is that 

 when the resistance increases from a pressure of, in round numbers, 

 10 mm. to 40 mm. of mercury, the flow decreases from 0"6 c.c. per 

 second to 0*2 c.c. per second, and the period of expulsion diminishes from 

 056 second to - 51 second, the systolic discharge consequently diminish- 

 ing from 033 c.c. to 0*08 c.c. As throughout the series of experiments 

 on which this statement is founded the initial impletion of the ventricle 

 was the same, it is obvious that the quantity of fluid which remains 

 when the valve closes, i.e. when the How through the aortic orifice 

 ceases, increases with the diminution of the systolic discharge. 



In the experiments just referred to, the element of elasticity is, for 

 the sake of simplicity, excluded. It is easy to modify the apparatus 

 used so as to include it. The effect of doing this is, that the attainment 

 of the maximum of intraventricular pressure is postponed, and that, in 

 connection therewith, the notch produced by the back-How from the 

 aorta at the moment that the valve closes is accentuated. 



The preceding considerations may be summarised as follows: — (1) The 

 isometric curve of muscle is represented by the pressure curve of the systole 



Two seconds. 



Fig. 248. — Isometric and isotonic curves of the ventricle of the heart of the frog 

 (given to the Avriter by Dr. O. Frank). The rate of movement of the recording 

 surface is indicated by the horizontal line, a, The isometric curve is the record 

 of a membrane manometer (Frank's) in connection with the cavity of the ven- 

 tricle, of which the impletion was constant, b, The isotonic curve is the record 

 of a Marey's capside in connection with a plethysmography chamber contain- 

 ing the heart. In the cavity of the ventricle the lateral pressure was constant, 

 and amounted to 10 cm. of water. 



of the charged ventricle with arrested discharge. (2) The isotonic curve of 

 muscle is represented by the volume curve of the ventricle ; the load, by the 

 resistance in front as measured by the aortic pressure. (3) As in muscle the 

 isometric curve culminates before the isotonic, so in the ventricle the isometric 

 pressure curve culminates sooner than the isotonic volume curve (Fig. 248). 

 The pressure curves and volume curves recorded in a ventricle working under 

 conditions which exist in the circulation, resemble in general the after-loading 

 curves of muscle. If the arterial pressure during the discharge is constant, 

 the resemblance is complete. The want of this constancy in the circulation 

 is dependent on conditions which lie outside our present inquiry. 



