THE MUSCULAR SYSTEM 61 



a short one, and the part to which it is attached is moved 

 farther. When the load is increased, the extent of contraction 

 is diminished. 



The speed and strength of the contractile power depend on the 

 tonicity of the muscles, their length, and their massiveness. A 

 horse with long legs can develop greater speed than one with 

 short legs, provided the muscles of each are of equal tone. This 

 is due to the greater length of the tendons which act to better 

 advantage on the bony levers. On the other hand, if two muscles 

 of the same length and power of contraction are compared, the 

 greater strength will be developed by the one with the more 

 volume. In other words, the strength of a muscle is in proportion 

 to its thickness or massiveness. From this discussion we may 

 summarize as follows: Muscles of strength are short and thick, 

 those of speed are long and slender. These facts are clearly 

 demonstrable by comparing the draft horse, which has heavily 

 developed muscles and wonderful ability to pull heavy loads, 

 with the race-horse which has long, slender muscles, and limbs 

 that give a wide range of action and develop a much higher rate 

 of speed. 



THE CHEMICAL COMPOSITION OF MUSCLES 



The chemical elements of which the muscles are composed in 

 general are the same as those of the entire body (see Chapter I). 

 They consist for the most part of carbon, hydrogen, oxygen, 

 nitrogen, sulphur, and phosphorus in various combinations. 



The inorganic compounds of muscles are water, which comprises 

 about 75 per cent, of fresh muscle, and salts, which exist chiefly 

 in the form of the potassium salts of phosphoric acid. Small 

 traces of calcium, magnesium, and iron are also present, but 

 are of minor importance as they are in most other tissues except 

 bone. 



The organic compounds of muscles consist of a variety of 

 substances, the most important of which are the following: 

 (1) Muscle proteins, which constitute about 20 per cent, of muscle 

 tissue, are found as myosin and.myogen. They may be obtained 

 for study by expressing the juice or plasma from muscle. (2) 

 Carbohydrates are in the form of glucose and glycogen. The pres- 

 ence of considerable sugar in horse meat gives it a peculiar 

 sweetish taste. (3) Fat is found in very small quantity in the 



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