438 PROCESSES INFERRED FROM INDIRECT OBSERVATION 



be advanced. The cane-sugar may be in truth the first sugar to be 

 synthesized, or, on the other hand, glucose may be the first sugar 

 formed, levulose arising from it by a transformation which can be 

 accomplished in laboratory-glassware, and the constancy of the hexose 

 percentage may merely mean that hexose in excess of this amount is 

 condensed to cane-sugar as rapidly as it is formed. It should be 

 mentioned, however, that the formation of levulose from glucose by 

 alkalies in laboratory-glassware is accompanied by the simultaneous 

 formation of Mannose, which sugar is absent from foliage- leaves. On 

 the other hand no laboratory-method of directly deriving cane-sugar 

 from formaldehyde has yet been discovered. 



THE CONVERSION OF CHEMICAL INTO MECHANICAL ENERGY: 

 THE CHEMICAL MECHANICS OF MUSCULAR CONTRACTION. 



We have seen that upon a normal mixed diet the necessary energy 

 for the performance of muscular work is derived from the oxidation of 

 Carbohydrates and that the final products of this oxidation are carbon 

 dioxide and water, an intermediate stage of the combustion being the 

 formation of Lactic Acid. So much we can ascertain by methods of 

 direct analysis. If we desire, however, to complete the story of the 

 energy-cycle which begins with photosynthesis in the plant, and cul- 

 minates in the release of heat and mechanical work by the animal, 

 purely analytical methods will not suffice and we are impelled to seek 

 additional information by the method of inference from indirect 

 observation. 



Our object is to ascertain the nature of the chemical machine which 

 transforms the potential energy of carbohydrates into muscular work 

 and heat. This problem divides itself into two parts, namely the 

 question of the nature of the process of combustion and the question 

 of the means of transforming the energy which combustion releases 

 into mechanical work. 



In respect to the first of these questions, it has long been a familiar 

 fact that when a muscle is repeatedly stimulated, either directly or 

 indirectly through its motor-nerve, the first few contractions gradually 

 and with considerable regularity increase in height until they reach a 

 maximum for a given strength of stimulus. This phenomenon to 

 which the name of "treppe" or the "Staircase Phenomenon" was given 

 by Bowditch, has been the subject of considerable investigation and 

 conjecture. Of a similar nature is the phenomenon of "Summation of 

 Stimuli," whereby a stimulus of strength insufficient to give rise to a 

 response when it is first applied, may be made, by repetition, to elicit 

 a response. 



It is to Waller that we owe the suggestion that the "staircase" is, 

 in reality, due to the increased production of carbon dioxide by the 

 contractile or conducting tissue. He observed that small amounts of 

 carbon dioxide augment the electrical response of nerve-fibers to 



