18 THE HUMAN BODY. 



certain properties which are distinctive of all living things 

 as compared with inanimate objects. 



Cell Growth. In the first place, each such cell can take 

 up materials from its outside and build them up into its 

 own peculiar substance; and this does not occur by the 

 deposit of new layers of material like its own on the surface 

 of the cell (as a crystal might increase in an evaporating 

 solution of the same salt) but in an entirely different way. 

 The cell takes up chemical elements, either free or com- 

 bined in a manner different from that in which they exist 

 in its own living substance, and works chemical changes 

 in them by which they are made into part and parcel of 

 itself. Moreover, the new material thus formed is not de- 

 posited, at any rate necessarily or always, on the surface 

 of the old, but is laid down in the substance of the already 

 existing cell among its constituent molecules. The new- 

 formed molecules therefore contribute to the growth of the 

 cell not by superficial accretion, but by interstitial deposit 

 or intussusception. 



Cell Division. The increase of size, which may be 

 brought about in the above manner, is not indefinite, but 

 is limited in two ways. Alongside of the formation and 

 deposit of new material there occurs always in the living 

 cell a breaking down and elimination of the old; and when 

 this process equals the accumulation of new material, as it 

 does in all the cells of the Body when they attain a certain 

 size, growth of course ceases. In fact the work of the cell in- 

 creases as its mass, and therefore as the cube of its diame- 



Fia. 6. A white blood corpuscle dividing, as observed at successive intervals 

 of a few seconds with the microscope. 



ter; while the receptive powers, dependent primarily upon 

 the superficial area, only increase as the square of the di- 

 ameter. The breaking down in the cell increases when its 



