210 VETERINARY PHYSIOLOGY 



free, but as part of the muscle complex. It is more 

 abundant in white than in red muscle, and when muscle 

 degenerates it decreases in amount. How it is ' formed is 

 not known, but it may be produced from methyl-guanidin 

 by union with acetic acid, methyl-guanidin being toxic while 

 creatin is inert. The fate of creatin will be considered 

 later. 



Purin Bodies (see Appendix), in the form of the amino- 

 purins, adenin and guanin, are present in small quantities, 



7. The Colour of Muscle varies considerably, according to 

 the preponderance of one or other type of fibre, some muscles 

 being very pale, almost white in colour — e.g. the breast 

 muscles of the fowl ; others again being distinctly red, even 

 after all the blood has been removed. This red colour is, in 

 some cases, due to the presence of the pigment of blood, 

 hcBVioglohin, but in certain muscles it is due to a peculiar 

 set of pigments, unyohcematins, giving different reactions from 

 the blood pigment. 



8. Inorganic Constituents. — 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. 



III. Physical Characters and Physiology. 



(1) Muscle is translucent during life, but, as death-stiffen- 

 ing sets in, it becomes more opaque. 



(2) Muscle is markedly extensile and elastic. A small 

 force is sufficient to change its shape, but, when the distort- 

 ing force is removed, it returns completely to its original 

 shape, provided always that the distortion has not over- 

 stepped the limits of elasticity. 



When a distorting force is suddenly applied to muscle — 

 e.g. if a weight is suddenly attached — the distortion takes 

 place at first rapidly, and then more slowly, till the full 

 effect is produced. If now the distorting force is removed, 



