THE PROGRESS OF S( n:\ n 



clear and distinct, though by no means complete, 

 conceptions of the manner in which the great 

 functions of assimilation, respiration, secretion, 

 distribution of nutriment, removal of waste pro- 

 ducts, motion, sensation, and reproduction are 

 performed ; while the operation of the nervous 

 system, as a regulative apparatus, which influences 

 the origination and the transmission of manifesta- 

 tions of activity, either within itself or in other 

 organs, has been largely elucidated. 



I have pointed out, in an earlier part of this 

 essay, that the history of all branches of 

 science proves that they must attain a consider- 

 able stage of development before they yield 

 practical " fruits ; " and this is eminently true 

 of physiology. It is only within the present epoch, 

 that physiology and chemistry have reached the 

 point at which they could offer a scientific foun- 

 dation to agriculture; and it is only within the 

 present epoch, that zoology and physiology have 

 yielded any very great aid to pathology and hy- 

 giene. But, within that time, they have already 

 rendered highly important services by the explor- 

 ation of the phenomena of parasitism. N.M only 

 the history of the animal parasites, such as 

 the tapeworms and the trichina, which intes; 

 and animals, with deadly results, been cleared up 

 by means of experimental investigations, and effi- 

 cient modes of prevention deduced from the data 

 so obtained ; but the terrible agency of the para- 



