930 SPECIAL PHYSIOLOGY. 



exception ; but this is not entirely so ; it is only the case when motion 

 is communicated to external matter. At the moment of action of a 

 muscle, indeed, an inverse proportion exists between the work accom- 

 plished and the heat produced. A muscle develops more heat, when it 

 cannot shorten itself, so as to produce external movement or work ; as, 

 for example, when a person attempts to move an overwhelming weight, 

 or overcome an unyielding resistance, as compared with the effect of 

 free action in lifting a movable weight. Any effort or motion which 

 is stopped or resisted, or which disappears in any way, passes neces- 

 sarily into heat ; even the electric currents in muscles and nerves, 

 when they are lessened by disturbance or rest, contribute, however 

 slightly, to raise the temperature of an acting muscle, and therefore 

 of the body. In fever, the muscles may become as hot as 105 Fahr., 

 and in tetanus, 111 Fahr. (Ludwig) ; they are then hotter even than ' 

 the blood (Fick) ; but this happens when most of their chemical 

 energy of decomposition can pass into heat, none being required for 

 work. During muscular action, the chemical energy passes into that 

 perceptible motion which we call contraction or shortening; during 

 arrested effort, it appears in the invisible motion, which produces heat. 

 On the supposition that the muscular force is derived from the oxi- 

 dation of albuminoid substances, the greater part, or the whole of it, is 

 ultimately transformed into heat, and is added to the avowedly larger 

 store derived from the non-nitrogenous food. But the theory which 

 regards animal motion as chiefly, or entirely, derived from the energy 

 supplied by the non-nitrogenous materials of the blood or tissues, and 

 therefore from the non-nitrogenous food, is not inconsistent with the 

 view, that these latter are the calorific or heat-forming materials; for 

 they then serve both offices. The transformation of potential energy 

 into muscular power, whether exerted internally or externally, is 

 necessarily accompanied by the ultimate production of heat within the 

 body ; and this is the chief, and probably the only, source of animal 

 heat. (Frankland.) 



Nutritive or Assimilative Work. 



The assimilative work performed in the body is also chemical, being 

 partly liquefacient, partly dialytic, and partly solidifacient. It must 

 be performed at the expense of chemical energy, developed during the 

 many transformations of the nutritive materials, as these are in turn 

 digested, hydrated, dissolved, absorbed, and converted into tissue. The 

 amount of force employed in digestion is small, as compared with the 

 other great demands of the system. Playfair suggests that it is 

 measurable by the amount of nitrogen of the nitrogenous substances 

 found in the solid excreta ; and that it may, as he thought of the 

 mechanical work, be ultimately referable to the energy of the albu- 

 minoid food. The quantity of nitrogen which escapes by the lungs 

 and skin is quite unimportant ; with ordinary diet, the urea includes 

 { Jths of that contained in the food, whilst the solid excreta yield about 

 y'jth. (Ranke.) This small quantity is the residue of the mucus, sali- 

 vin, pepsin, pancreatin, glycocoll, and taurin of the digestive fluids, 



