150 
MESSRS. W. H. HOWELL AND E. DONALDSON 
By the investigations of Weber, Heidenhain, Fick, and others, it has been proved 
for ordinary skeletal muscles that the work done in contracting, measured by the 
product of the load into the lift, increases up to a certain limit with the load to be 
raised. The increase in work, however, is not proportional to the increase of load, 
since as a general rule the lift is diminished as the load is increased, except, perhaps, 
for minimal weights. If, now, aortic pressure is taken as the equivalent of the load 
which an ordinary muscle raises when it contracts, the law given above for the work 
of the left ventricle may be expressed in the terms of muscle physiology in this way. 
The work done by the heart muscle when it contracts, measured by the product of the 
weight of blood ejected at each contraction into the height of aortic pressure, not only 
increases with the Joad against which it contracts, but increases in direct proportion to 
the load, within the limits given. It is not probable that this proportional increase of 
work by the heart muscle is owing to any nervous mechanism co-ordinating the discharge 
of energy with the resistance to be overcome. Considering it as a muscle phenomenon 
alone, two explanations suggest themselves. It might be conceived that within the 
limits given, the total energy liberated at each contraction remains constant, and that 
that portion of it which is not used up in external work disappears as heat liberated in 
the heart itself. So that as the arterial pressure increases, making the resistance to be 
overcome greater, a correspondingly greater portion of the energy appears as external 
work, and vice versa. Outside of the ■waste of energy which such a supposition 
involves, the study of the development of heat in a contracting skeletal muscle teaches 
us that for it, at least, there is no such inverse proportion between the amount of heat 
liberated and of external work done in a muscle contracting under different loads. The 
curves of heat development and of mechanical work, on the contrary, follow a somewhat 
similar course. An explanation more in accordance with what is known of the 
physiology of ordinary muscle is found in the supposition that as the load increases a 
greater amount of energy is liberated, in consequence of some change in the molecular 
state of the ventricular muscle associated with increased tension at the commencement 
and during the early stages of its contraction. This is the supposition adopted for 
ordinary muscle ; as Foster states it, “the tension of the muscular fibre increases the 
facility with which the explosive changes resulting in a contraction take place ” (Physiol., 
1883, p. 88). The difference between the two muscles is that for the heart muscle 
the energy liberated as external work bears a direct proportion to the tension exerted 
by the load, while for ordinary muscle this is not true. The lift of an ordinary 
muscle when contracting is represented, in the case of the heart muscle, by the extent of 
the contraction, measured by the volume of blood ejected. Since within the range of 
arterial pressures given the volume of blood thrown out at each systole remains the 
same, it follows that the extent of the contraction is unchanged. The most probable 
interpretation of this fact is that the contraction in each case is maximal, and 
completely empties the ventricular cavity; a conclusion which is in accordance with 
the work of Bowditch, Kronecker, and others on the isolated Frog’s heart. 
