The Muscular System. 243 



believe that during the life of the muscle it does not exist 

 as myosin, any more than fibrin exists in living blood as 

 fibrin. The living muscle therefore contains myosinogen, 

 or myosin precursor, bearing the closest resemblance to the 

 fibrin of the blood. 



Myosin only differs from the blood fibrin by being soluble 

 in a dilute solution of common salt and dilute hydrochloric 

 acid. 



The muscle serum consists of three proteids, two being 

 globulins and one an albumin ; it possesses an acid re- 

 action due to the presence of lactic acid, the result of the 

 process of coagulation. 



In muscle is also found small quantities of volatile fatty 

 acids, such as acetic and butyric, and an organic acid, 

 lactic, which exists in two forms. The importance of this 

 latter acid in the production of rigor mortis, or muscle 

 coagulation, will be dealt with presently. Carbo-hydrate 

 substances, such as glycogen and sugar, are also found in 

 muscles ; their function is the liberation of muscular 

 energy. The muscles of the horse contain a considerable 

 quantity of glycogen even after starvation (see p. 166). 



Certain extractives can be obtained from muscular tissue, 

 such as creatin, creatinine, carnin, a variable amount of 

 glycogen and sugar, together with a trace of urea, and small 

 quantities of muscle sugar (inosite), alcohol, and lactic acid. 



The salts of muscle are principally those of potassium. 



A quantity of carbonic acid gas can be obtained from 

 muscle, together with a small proportion of nitrogen, but 

 no oxygen. 



Changes resulting from Contraction. — The changes occurring 

 in muscles are remarkably active ; the so-called explosions 

 which result in muscular contraction, use up at every 

 moment the combustible material of the structure, and the 

 products arising from their combustion have to be got rid 

 of at once and repair brought about ; but changes are con- 

 stantly occurring even during the period of muscle rest. 

 Muscle activity is characterized by muscle waste ; muscle 

 rest is characterized by a preponderance of the process 



16—2 



