INTRODUCTION. m 47 



liquid should be produced, vessels proper for the 

 purpose are found floating in the nutritive fluid, 

 and propel through their pores the elements requi- 

 site for the composition of such liquid. 



Thus it is that the blood incessantly renovates 

 the composition of the various parts of the body, 

 and restores it to its original state, from those losses 

 and alterations which are the necessary conse- 

 quences of the exercise of the animal functions. 

 We can form a tolerably clear idea of this opera- 

 tion in the general, though our insight to its de- 

 tails is very indistinct, and though our ignorance 

 of the precise proportions of chemical composition 

 of each individual part prevents us from learning 

 with exactness the transmutations which are neces- 

 sary for its sustentation. Besides the glands, which 

 separate from the blood those liquids necessary to 

 the internal economy of the animal system, there 

 are others which secrete such fluids as are destined 

 for rejection. 



In the function of generation there is one pheno- 

 menon, which presents us with a difficulty of a very 

 different description from that which secretion offers 

 to our conception. This phenomenon is the pro- 

 duction of the germ, and in truth, it must be re- 

 garded as almost incomprehensible. Admitting, 

 however, its existence, without attempting to ac- 

 count for it, we shall experience no particular diffi- 

 culty in tracing the remaining part of the process of 

 generation. As long as the germ adheres to the 

 parent, it is nourished as an integral part of that 



