114 THE HUMAN BODY 



rounding sarcoplasm brings about a swelling of the sarcostyles, 

 which, by virtue of their peculiar segmental structure, results in 

 turn in a forcible contraction. The shortening of the whole 

 muscle is nothing more than the sum total of the contractions of 

 the individual sarcostyles. Relaxation is brought about through 

 the removal of the lactic acid, immediately by diffusion, but 

 ultimately by being rebuilt into precursor at the expense of energy 

 obtained through further oxidation of fuel. 



Oxidation in Muscle. Reference has already been made (p. 104) 

 to the fact that in the muscle oxidation occurs at about the tem- 

 perature of the body, instead of at the high temperature character- 

 istic of ordinary combustion. This is true of all oxidations in 

 living cells. It is accomplished through the presence of special 

 substances known as oxidases, belonging to the class of enzyms 

 (p. 14). These have the property of bringing the oxygen and the 

 fuel into such intimate relationship that they will combine chem- 

 ically without first having to be heated to a high temperature. 



Hormone of Skeletal Muscle. A curious fact about muscle is 

 that the oxidation of sugar within it is subject to the control of a 

 hormone. Why this control should exist is not clear, but that it 

 does exist is proven beyond doubt. The hormone is secreted by 

 certain masses of cells which are embedded in one of the digestive 

 glands, the pancreas (p. 460). The importance of the hormone is 

 shown by the dire results that follow its absence. A well-known, 

 and unfortunately rather common disease, diabetes, is caused by 

 the failure of these cell masses in the pancreas to manufacture 

 their hormone in normal amounts. The muscles thereupon lose 

 in greater or less degree the power to utilize sugar as fuel, and 

 suffer, in consequence, more or less serious impairment of function. 

 The unused sugar accumulates in the blood and is discharged 

 through the kidneys, giving rise to the most conspicuous symptom 

 of the disease, sugar in the urine. This condition is discussed in 

 greater detail in a later chapter (p. 496). 



Physiology of Smooth Muscle. Smooth muscle differs strik- 

 ingly from skeletal muscle, not only, as stated previously, in struc- 

 ture, but also in mode of action. Aside from the fact that both 

 sorts of muscle produce their effects by contraction they have 

 almost no features in common. Weight for weight smooth muscle 

 is much less powerful than skeletal muscle. Its movements are 



