CELLS. 17 



as some coals are burnt in the furnace of an engine for 

 every bit of work it does; in the same way every thought 

 that arises in us is accompanied with the destruction of 

 some part of the Body. Hence these primary actions of 

 keeping warm, moving, and being conscious necessitate 

 many others for the supply of new materials to the tis- 

 sues concerned and for the removal of their wastes; still 

 others are necessary to regulate the production and loss of 

 heat in accordance with changes in the exterior tempera- 

 ture, to bring the moving tissues into relation with the 

 thinking, and so on. By such subsidiary arrangements 

 the working of the whole Body becomes so complex that it 

 would fill many pages merely to enumerate what is known 

 of the duties of its various parts. However, all the proper 

 physiological properties 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 manifestations 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 liistologist meets with as entering 

 into the composition of the human Body 

 are minute granular masses of a soft con- 

 sistence, about 0.012 millimeter (y^To f an 

 inch) in diameter (Fig. 5, e). Imbedded in 

 each lies a central poitim, not granular 

 and therefore different in appearance from 

 the rest. These anatomical units are known FIG 5 _ Formg 

 as cells, the granular substance being the of cells from the 



iJodv 



cell body and the imbedded clearer portion 

 the cell nucleus. Inside the nucleus may often be distin- 

 guished a still smaller body the nucleolus. Cells of this 

 kind exist in abundance in the blood, where they are known 

 as the -white Hood corpuscles, and each exhibits of itself 



