120 METABOLISM. 



This change is called hydrolysis, and the action is said to be 

 kydrolytic. Chemistry has established this fact for amylolytic and 

 inversive enzymes, and it is probably equally true for those that 

 are proteolytic. For the action of all enzymes the presence of 

 water is essential. 



The consideration of the individual enzymes will be deferred 

 until the action of the various fluids in which they occur is dis- 

 cussed. 



METABOLISM. 



The human body is during life the seat of constant activity, 

 during which almost limitless chemical changes take place. These 

 are collectively spoken of under the term metabolism. Some 

 of these are concerned with the upbuilding of the body, and 

 are termed anabolic; while others result in the wasting of 

 the tissues, and are denominated katabolic. Anabolism and 

 assimilation may be regarded as synonymous terms, while katab- 

 olism and destructive assimilation express somewhat the same 

 idea. The word metabolism has been defined as "the process 

 by which living cells or organisms are capable of incorporating 

 substances obtained from food into an integral part of their own 

 bodies ; the changes that proteids and other constituent substances 

 undergo in the body. It is constructive when the substance be- 

 comes more complex ; destructive or retrograde when it becomes 

 simpler by the change." Still another lexicographer defines metab- 

 olism as " The act or process by which, on the one hand, the dead 

 food is built up into living matter, and by which, on the other, 

 the living matter is broken down into simple products within a cell 

 or organism ; the sum of the anabolic or constructive (assimilation) 

 and the katabolic or destructive (decomposition) processes." If 

 the anabolic and katabolic processes should exactly balance one 

 another, the body would be in a state of equilibrium, but this never 

 occurs absolutely. 



As a result of the destructive changes which take place in 

 the body it is essential that they be counterbalanced so far as is 

 possible, and this is accomplished by taking into the body food 

 and oxygen. If an individual is deprived of oxygen, death occurs 

 in a few minutes from asphyxia. No less certainly does death 

 supervene if he is deprived of food, although the time required to 

 bring about the result is much greater, depending considerably 

 upon the circumstances and upon the age of the individual. In 

 the instance frequently quoted, when, in the year 1816, one hun- 

 dred and fifty persons were wrecked on the " Medusa," all but 

 fifteen were dead after having been without food, either solid or 

 liquid, for thirteen days. In this instance, however, it must be 

 borne in mind that the exposure incident to the shipwreck proba- 

 bly contributed to hasten the fatal result. It may be said, in 

 general, that death will supervene when the body has lost four- 



