148 THE BODY AT WORK 



long period of starvation. The muscles lose during activity 

 he glycogen which they contain when at rest. 



It has already been pointed out that the body is not entirely 

 dependent upon external agencies for the production of the 

 sugar which it needs. When the supply is inadequate, it manu- 

 factures glycogen for itself out of the other constituents of 

 the diet. It can, indeed, make it at the expense of its own 

 proteins. If a dog which has been caused to do muscular 

 work, without a sufficiency of carbohydrate food, until (as 

 judged from a control experiment) all glycogen has disappeared 

 from its liver, be placed under the influence of a narcotic drug, 

 which arrests the activity of its muscles, glycogen reappears. 



Dietetics. Even those who are most ignorant of the science 

 of physiology flatter themselves that they have one piece of 

 information : " The whole of the body is renewed once in every 

 seven years." I cannot trace the origin of this sapient apo- 

 thegm, which for generations has passed current. If seven 

 weeks or seventy years were the period allowed for the renewal 

 of the tissues, the statement would be equally near the truth. 

 Judging from the rate at which they are destroyed, it is unlikely 

 that blood-corpuscles live for more than five or six weeks. 

 Hairs are shed about two years after they first appear above the 

 surface. On attaining this age a hair drops off and a new one 

 takes its place. The superficial cells of the skin are shed in 

 great numbers every day, and their place taken by younger 

 cells which come up from the deeper layers. The cells of many 

 glands would seem to have a comparatively short term of life. 

 On the other hand, some tissue-elements are far more perma- 

 nent. By the time a child is a year old all its nerve-cells are in 

 position. They last as long as the individual lives. When 

 the statement with regard to the renewal of the tissues is 

 understood as meaning, not that the cells are destroyed and 

 replaced by new ones, but that within a period of seven years 

 all the molecules which enter into their protoplasm are ex- 

 truded from the body and replaced by molecules received as 

 food, the assertion verges on the transcendental. It is unlikely 

 that we shall ever obtain data against which it can be checked. 



The essential part of every living cell is its spongework of 

 protoplasm. "Bioplasm " is perhaps a better term to use when 

 we are speaking of protoplasm as a structure, since it does not 



