78 TEXT-BOOK OF PHYSIOLOGY 



become shorter and wider (page 53). He therefore assumed that some 

 of the heat liberated acting on the doubly refracting material caused it to 

 imbibe water from the light band. The cylindrical contractile element was 

 thus caused to shorten and widen and assume a more or less spherical 

 shape. During the subsequent cooling of the muscle, the reverse process 

 took place. This explanation is supported by phenomena exhibited 

 by other doubly refracting material under similar conditions. Thus a 

 piece of catgut when supported and attached to a recording lever and 

 immersed in water, will shorten when suddenly heated, and relax when 

 cooled, recording a curve similar to that recorded by a muscle. This view 

 has been adversely criticised by reason of the fact that the high temperature 

 necessary to give to the muscle the efficiency it ordinarily exhibits, would 

 destroy the muscle material and at a low temperature the ordinary 

 efficiency could not be attained. The muscle therefore can not be looked 

 on as Englemann supposed as a machine capable of directly transforming 

 heat into mechanical work as is the case with a gas or a steam engine. 



Hill has recently offered another explanation based on a great variety 

 of accurate experiments and calculations. This investigator assumes that 

 there is primarily in the muscle a complex compound the exact nature 

 of which has not been determined but which contains potential energy. 

 The arrival of the nerve impulse leads to a disruption of this compound 

 with the liberation of lactic acid. The acid at once acidifies the sarcous 

 elements and in so doing endows them with the power of imbibing water 

 from the sarcoplasm, whereupon they swell and tend to approximate 

 a spherical shape and thus shorten the muscle. Following the contraction 

 or the shortening, there occurs an oxidation of sugar with the production 

 of carbon dioxid and water and the liberation of heat. A portion of the 

 heat is then utilized in the re-formation or reconstruction of the compound. 

 The lactic acid is again incorporated in whole or in part and the heat 

 absorbed is transformed into potential or chemical energy. With the 

 withdrawal of the acid, the sarcous elements lose their imbibition power 

 and the fluid returns to the surrounding sarcoplasm. 



THERMIC PHENOMENA 



The potential energy liberated in a muscle on the arrival and subsequent 

 action of a nerve impulse, manifests itself partly as heat and partly as 

 mechanic motion or a change of shape of the muscle. Though heat pro- 

 duction is taking place even during the passive condition, it is largely in- 

 creased by muscle activity. The amount of heat produced will vary however 

 with a variety of conditions, as strength of stimulus, tension, work done, etc. 



Stimulus. It has been experimentally determined that the skeletal 

 muscle of the frog, the gastrocnemius, shows after a single contraction a rise 

 in temperature of from o.ooiC. to o.oo5C. and after tetanization an 

 increase of from o.i4C. to o.i8C. It has also been shown that an increase 

 in the strength of the stimulus from a minimal to a maximal value increases 

 the amount of heat liberated. This is the direct result of increased chemic 

 change naturally following increased stimulation. 



Tension. The greater the tension of a muscle, the greater, other condi- 

 tions being the same, is the amount of heat liberated. If the muscle is 



