36 THE BODY AT WORK 



units does not hold good to the same extent when they 

 are viewed from a physiological standpoint. A cell is an 

 administrative area. For purposes of nutrition, respiration, 

 and cell division it is autonomous. It is responsible for its 

 own' local affairs. If a part is cut off from it, this part loses 

 its vitality ; this, at least, is the conclusion drawn from the 

 atrophy of the axons of nerves when they are cut off from 

 the cells of which they are outgrowths. Apparently we must 

 understand by " the cell," when speaking of the cutting off 

 of a part, the portion of the cell which retains the nucleus ; 

 although we must be careful not to lay too much stress upon 

 the nucleus as the centre of cell life. Red blood-corpuscles, as 

 already pointed out, have no nuclei, and yet they live. Cell 

 growth, estimated by mere increase in size, does not depend 

 upon the nucleus. Many cells of the skin and its appendages 

 increase considerably after the nucleus shows changes which 

 clearly indicate that it is far advanced towards decay. But 

 increase in protoplasm, cell growth in a legitimate sense, 

 and especially cell division, are dependent upon the presence 

 of an active nucleus. While, therefore, histologists no longer 

 formulate the cell theory in the restricted terms in which ifc 

 was enunciated some years ago, they still regard the cell as 

 the unit of structure and unit of function. The body is built 

 of cells, and whatever is done by the body as a whole is done 

 by its individual cells. 



