A text-book of physiology : for medical students and physicians / by William H. Howell.

  • Howell, William H. (William Henry), 1860-1945.
Date:
1907
Catalogue details

Licence: Public Domain Mark

Credit: A text-book of physiology : for medical students and physicians / by William H. Howell. Source: Wellcome Collection.

Provider: This material has been provided by the Augustus C. Long Health Sciences Library at Columbia University and Columbia University Libraries/Information Services, through the Medical Heritage Library. The original may be consulted at the the Augustus C. Long Health Sciences Library at Columbia University and Columbia University.

    48/1044 (page 40)
    curve has the general form indicated in Fig. 18. Three facts are expressed by this curve: First, that if the muscle lifts no weight no work will be done; this follows theoretically from the formula W = LH, in which W represents the work done, L the load, and H the lift. If either L or H is equal to zero the product, of course, is zero; that is, no external work is done; the chemical energy liberated in the contraction takes the form of heat. Under such circumstances the amount of heat given off from the muscle should be greater than when a load is lifted. In accordance with this fact it is found that a muscle lifting a light load gives off more heat during the contraction than when lifting a heavier load. Second. There is an optimum load for each muscle with which the greatest proportion of work can be obtained. Third. When the load is just sufficient to counteract the contraction of the muscle no work is done, H in the above formula being zero. This amount of load measures what Weber called the absolute power of the muscle. As will be seen from the above curve, it is measured by the weight which the muscle cannot lift and which, on the other hand, cannot cause any extension of the muscle while contracting. Or, in more general terms (Hermann), the absolute power of a muscle is the maximum of tension which it can reach without alteration of its natural length. This absolute power can be measured for the muscles of different animals and for convenience of comparison can then be expressed in terms of the cross-area of the muscle given in square centimeters. Weber has shown that the absolute power of a muscle varies with the cross-area, since this depends upon the number of constituent fibers whose united contraction makes the contraction of the muscle. Expressed in this way, it is found that the absolute power of human muscle is, size for size, much greater than that of frog's muscle. For in- stance, the absolute power of a frog's muscle of 1 square centimeter cross-area is estimated at from 0.7 kilogram to 3 kilograms, while that of a human muscle of the same size is estimated by Hermann at 6.21 kilograms. Taken as a whole, the human muscle is a better machine for work, but it seems possible, although exact figures are lacking, thai the absolute power of the muscles of some insects reckoned for the same unit of cross-area would be much greater than in human muscle. COMPOUND OR TETANIC CONTRACTIONS. Definition of Tetanus —When a muscle receives a series of rapidly repeated stimuli it remains in a condition of conlTaction as long :i- ilf stimuli are sent, in or until it loses its irritability from the effecl of fatigue. A contraction of this character is described compound contraction or tetanus. If the stimuli follow each