THE TISSUES 39 



2. Chemistry of Muscle 



Like all other living tissues, muscle ; is largely composed of 

 water. It contains about 75 per cent. The 25 per cent, 

 of solid constituents is made up of a small quantity, about 

 3 per cent., of ash, and 22 per cent, of organic substances. 

 The ash consists chiefly of potassium and phosphoric acid, with 

 small amounts of sulphuric and hydrochloric acids and of sodium, 

 magnesium, calcium, and iron. The sulphuric acid is derived 

 from the sulphur of the proteins, and a part of the phosphoric 

 acid is derived from the phosphorus of the nucleins of muscle, 

 and probably from other organic combinations. 



1. Proteins. Of the organic constituents, by far the greater 

 part is made up of Proteins. These may be divided into 



(a) Those soluble in neutral salt solutions. 



(b) Those insoluble in them. 



(a) The first class of bodies consists entirely of three 

 globulins. Two of these Myosinogen and Paramyosinogen 

 have the peculiar property of clotting under certain condi- 

 tions, to form what is called Myosin, and this process, which 

 occurs after death, is the cause of death stiffening. The 

 post-mortem change is supposed to be brought about by the 

 ^development of an enzyme, since a glycerine extract of dried 

 muscle rapidly causes the formation of myosin. The third 

 globulin, Myoglobulin, does not undergo this change. These 

 three proteins are contained in the plasma the juice which 

 can be expressed from muscles kept near the freezing point. 

 If the plasma is warmed it rapidly clots, just as it does 

 post-mortem. 



(6) The insoluble protein of muscle, Myostromin, seems 

 to be of the nature of a nuclein, and probably forms the 

 framework of the fibres. It is always mixed with the collagen 

 of the fibrous tissue of muscle, and it may be separated by 

 dissolving it in carbonate of soda solution, from which it 

 may be again precipitated by weak acetic acid. (Chemical 

 Physiology.) 



Collagen derived from the fibrous tissue holding the muscle 

 fibres together is also present, and yields gelatin on boiling. 



