56 ANIMAL PHYSIOLOGY. 



is still contractile is neutral or slightly alkaline, except after 

 being thrown into a state of spasm. 



Although muscle-fibrin is the principal solid constituent 

 of muscle, there are numerous others which occur in small 

 quantity in solution, some of them containing nitrogen and 

 others not. The nitrogenous substances referred to are all 

 of them much simpler in chemical constitution than the 

 albuminoids, and the most important of them is called 

 kreatin, of which it is sufficient to note that it and its allies 

 are of a composition less complex than gelatin, and more 

 complex than urea. Among the non-nitrogenous substances 

 found in muscles may be mentioned grape sugar, and another 

 variety of sugar called iiiosite, also lactic, butyric, and other 

 acids. These various substances, both those which contain 

 nitrogen and those which do not, are probably formed by 

 processes of decomposition incident to activity of the muscu- 

 lar fibre, for their quantity is greater in muscle whose 

 irritability has been exhausted by electric stimulus, than in 

 muscle which has been at rest. Thus the hind limbs of a 

 frog have been separated from the animal, and one of them 

 has been subjected to severe electric stimulus, while the other 

 has been left at rest; and the muscles of the stimulated 

 limb have yielded a notably larger amount of substance 

 soluble in alcohol than those of its fellow (Helmholtz). 



34. During life, however, muscular action is sustained by the 

 combustion not of nitrogenous material but of non-nitrogen- 

 ous. This has been proved by a variety of experiments, in 

 which persons have been kept for days on a weighed diet of 

 known composition, and their urine and other excreta have 

 been daily analysed ; and it has been found that en days on 

 which they took violent exercise, they lost no more nitrogen 

 than on days when they were at perfect rest. By other ex- 

 periments it is known that the amount of carbonic acid 

 given off by the lungs is greatly increased by exercise. A 

 muscle may, therefore, be compared with an engine which 

 consumes in its work, not its own substance, but fuel. This 

 fuel is carbonaceous material, which is converted with the 

 aid of oxygen into carbonic acid and water; while it is in the 

 intervals of rest that the proper substance of the muscle, 

 consisting of albuminoid material, undergoes growth and repair. 



