THE SOURCE OF MUSCULAR ENERGY. 379 



under the influence of the same exciting cause, is what exerts the 

 power of the contracting muscle. The intensity of this shortening or 

 contracting power has been approximately measured e. g., by ascer- 

 taining experimentally the weight necessary to prevent a muscle from 

 contracting under excitation.* The muscles are supplied with blood 

 by the fine ramifications of the arteries, and the blood is conducted 

 away again by the ramifications of the veins, the arterial blood los- 

 ing oxygen and taking up carbonic acid during its passage, as is the 

 case in the other tissues also. 



Regarding the composition of the muscular tissue, it may be simply 

 noted that the tissue itself is composed mainly of albuminoid material 

 (cell-contents) and of the substance of the connective tissue, which is, 

 like the albuminoids, composed mainly of carbon, hydrogen, oxygen, 

 and nitrogen, and in much the same proportions. Besides this, the 

 blood and lymph permeate the muscular tissue throughout, and cer- 

 tain non-nitrogenous substances, mainly glycogen, a substance resem- 

 bling starch or dextrine in composition and properties, are stored up 

 in the muscular tissue, and always found to be present. Certain other 

 simple compounds containing nitrogen are also present, and are con- 

 sidered to be decomposition products of the more complex albuminoids. 

 When the muscular contraction takes place, mechanical force may be 

 exerted which is produced at the expense of the force stored up as 

 potential chemical energy in the materials which serve as the fuel ma- 

 terial. This potential energy is set free or rendered active by the 

 chemical processes which there take place, and appears as work, as 

 sensible heat, or as electrical disturbances. 



Before we inquire as to the nature of these chemical processes, it 

 will be of advantage to glance briefly at the results of important in- 

 vestigations which have been made on this subject, as these form the 

 only safe data by which we may judge of the tenability of any theory. 

 It would be out of place here to attempt a full reference to the mass 

 of investigations and experiments which have been published, and 

 which bear on the topic under discussion. f We shall therefore simply 

 notice the principal facts which have been established as the results 

 of those investigations, and which are most pertinent to the matter in 

 hand. 



The experimental researches on this subject may be classified under 

 four heads : 1. The examination of the muscular tissue itself before 



* This value has been found in man at about 6,000 to 8,000 grammes per square cen- 

 timetre of cross-section of muscle (85 to 114 pounds per square inch) for the maximum 

 for voluntary contraction. It is of course evident that the intensity of the force exerted 

 varies with the kind and degree of excitation, so that too much dependence must not be 

 placed on any particular values thus obtained. They simply give an approximate value 

 for ordinary muscular activity. 



f Quite full references may be found in the excellent and quite recent text-books of 

 F. Hoppe-Seyler, u Physiologische Chemie," and of A. Gamgee, " Physiological Chemistry 

 of the Animal Body." 



