62 PEINCIPLES OF VETERINARY SCIENCE 



muscle fibers. (4) Organic extractives are present in various 

 combinations. The most plentiful of these is creatin, a product 

 of metabolism. To some of these extractives meat juice owes its 

 property to stimulate appetite; they are increased to some 

 extent during the ripening of meats. Others are probably re- 

 sponsible for the unpleasant odor of muscle tissue from certain 

 animals. (5) Sarcolactic acid occurs in small amount in all 

 living muscles. (6) Pigments peculiar to the muscles give them 

 a red color, but the presence of the hemoglobin or red coloring 

 matter of the blood is .partially responsible for the redness. 



THE CHEMICAL CHANGES OF MUSCLES 



Chemical changes of various kinds are constantly taking place 

 in living muscles. They lead to the formation of products that 

 act as poisons unless removed before accumulated to excess. 

 A normal resting muscle when isolated from the body gives an 

 alkaline reaction. After it has been stimulated and caused to 

 contract it becomes acid. This change in reaction cannot be 

 detected in the body because the alkaline tissue fluid, which 

 bathes the muscle-cells, at once neutralizes the acidity. The 

 two acids which are responsible are, carbonic and sarcolactic. 

 Carbonic acid (C0 2 ) arises from the oxidation of the carbon- 

 containing constituents of the muscle. Its formation is always 

 accompanied by the production of heat and involves an exchange 

 with oxygen. Sarcolactic acid (C 3 H 6 3 ) is a product of muscle 

 activity. Another chemical change that occurs during activity 

 is the gradual conversion of the glycogen, which is stored in the 

 muscles during rest, into glucose to supply energy. If the activity 

 is long-continued not a trace of glycogen remains. The only 

 other chemical change that need be mentioned is the production 

 of energy, which, although left for last consideration, is the most 

 important from an economic viewpoint. 



Muscular energy is exhibited either as heat production or as 

 work performance. Under controlled conditions both forms can 

 be measured quantitatively and expressed in definite units. The 

 unit of heat, known as the calorie, is the amount of heat necessary 

 to raise one gram of water one degree Centigrade. The unit of 

 work, known as the kilogrammeter, is the amount of work done 

 in lifting a kilogram to the height of one meter from the surface 

 of the earth against the attraction of gravity. The muscles 



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