18 THE HUMAN BODY 



depend in ultimate analysis on a small number of faculties which 

 are possessed by all living things, their great variety in the Human 

 Body depending upon special development and combination in 

 different tissues and organs ; and before attempting to study them 

 in their most complex forms it is advantageous to examine them 

 in their simplest and most generalized manifesta- 

 tions, as exhibited by some of the lowest living 

 things or by the simplest constituents of our own 

 Bodies. 



Cells. Among the anatomical elements which 

 the histologist meets with as entering into the 

 composition of the Human Body are minute 

 granular masses of a soft consistence, about 0.012 

 millimeter (^5 of an inch) in diameter (Fig. 5, 6). 

 Imbedded in each lies a central portion, not so 

 granular and therefore different in appearance from 

 the rest. These anatomical units are known as 

 cells, the granular substance being the cell-body 



FIG. 5. Forms ' . 6 



of cells from the and the imbedded clearer portion the cell-nucleus. 

 Inside the nucleus may often be distinguished a 

 still smaller body the nucleolus. Cells of this kind exist in 

 abundance in the blood, where they are known as the white blood- 

 corpuscles, and each exhibits of itself 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 ma- 

 terials 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 combined 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 deposited, 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 con- 

 stituent molecules. The new-formed molecules therefore contrib- 

 ute to the growth of the cell not by superficial accretion, but by 

 interstitial deposit or intussusception. 



