56 THE HOBSE IN HEALTH AND DISEASE 



CHEMISTRY AND PHYSIOLOGY OF MUSCLES 



A chemical analysis of fresh muscle-fiber shows that approxi- 

 mately 75 per cent, of it is water. The balance of the constituents 

 are principally proteins, carbohydrates and fats, nitrogenous waste 

 products, and inorganic salts. Muscle proteins, which make up 

 the largest part of the organic constituents, are myosinogen, myo- 

 globulin, myostromin, and collagen. The carbohydrates are in the 

 form of sugar and animal starch or glycogen. Muscular tissue 

 has the power to convert glucose into glycogen, which is stored in 

 the muscle until required for use during muscular activity. It has 

 been clearly demonstrated that long-continued activity causes the 

 glycogen to completely disappear. Fat is present in small quanti- 

 ties in the fibers. The nitrogenous extractives are of various kinds; 

 the one present in greatest amount is creatin. The inorganic 

 constituents of muscle are chiefly salts of potassium, sodium, cal- 

 cium, magnesium, and iron, the former predominating. 



Irritability and Contractility .^-Muscles possess both of these 

 powers in well-developed form. IrritabiUty is a sensitiveness to 

 stimulation, and is an attribute of all living tissues. Contractility 

 is normally due to a stimulus received from the central nervous 

 system through motor nerves. If the latter are severed, the 

 muscles are paralyzed. The skeletal muscles are, however, capable 

 of independent contraction. Contraction as a result of thermal 

 stimulation is seen when an animal is butchered — the cold air 

 coming in contact with the recently exposed flesh sets up a pro- 

 nounced twitching in the muscles. When a muscle contracts it 

 shortens in length and increases in width. A contracted muscle 

 soon relaxes. This is due largely to the action of antagonistic 

 muscles. 



The means used to make a muscle contract for experimental 

 study are: (1) chemical agents, like dilute mineral acids and 

 metallic salts; (3) mechanical stimuli, like striking, pinching, or 

 tearing the muscle; (3) thermic changes brought about by the 

 application of heat or cold; (4) electric stimulation from the use 

 of the galvanic battery or an induction coil. The duration, curve, 

 and strength of contraction may be demonstrated by applying these 

 stimuli under properly controlled conditions. 



The chief functions of muscles are to produce heat and to 

 perform work. Heat is created in greater amount by increasing 



