CHAPTEE XIY. 



POPULAR DISCOVERIES IN PHYSIOLOGY. 



Recluse, th' interior sap and vapor dwells, 

 In nice transparence of minutest cells. 



H. Brooke. 



But a heavenly sleep 

 That did suddenly steep 

 In balm my bosom's pain. 



Shelley. 



THE science of Physiology, which investigates the 

 complex phenomena of the motions, sensations, growth, 

 and development of organisms, is almost wholly the 

 product of the present century; but with the exception 

 of a few fundamental conceptions, it has been an almost 

 continuous growth by small increments, and offers few 

 salient points of popular interest, or which can be made 

 intelligible to the general reader. 



The first of the great fundamental conceptions re- 

 ferred to is the cell-theory, which was definitely estab- 

 lished for plants in 1838, and immediately afterward 

 for animal-structures. The theory is that all the parts 

 and tissues of plants and animals are built up of cells, 

 modified in form and function in an infinite variety of 

 ways, but to be traced in the early stages of growth, alike 

 of bone and muscle, nerve and blood vessel, skin and 

 hair, root, wood, and flower. And, further, that all 

 organisms originate in simple cells, which are almost 

 identical in form and structure, and which thus consti- 

 tute the fundamental unit of all living things. 



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