PHYSIOLOGY. 207 



mass seem to be produced in consequence of a certain structure, 

 with perhaps some other condition in the extreme vessels 

 through which the fluids pass ; and a part having such a struc- 

 ture, is called a gland or secretory organ, the function of 

 which, from the most obvious notion of the manner of it, is 

 called secretion. 



CCLXXVIII. The structure of the organ, and the man- 

 ner of its function, seem to me for the most part unknown ; at 

 least, what we know or suppose with regard to the structure 

 hardly in any case applies to the explanation of the function. 



CCLXXIX. If it any how appeared that the several se- 

 creted fluids were all of them previously existent in the same 

 forms in the mass of blood, it would not perhaps be difficult to 

 explain what might be strictly called a secretion. But such 

 previous existence does not appear ; for, except the matter of ex- 

 halation into the several cavities of the body, and the matter of 

 urine and of perspiration, we find no proper evidence of any 

 other secreted fluids present in the mass of blood. We cannot 

 find there, either milk, mucus, or oil, and much less the appear- 

 ance of many other fluids, which are only found after they have 

 passed through certain organs. 



CCLXXX. This being the case, the considerations of the 

 physiologists with regard to the velocity of the blood, and other 

 circumstances favouring the separation of the parts of a fluid 

 which are only diffused among one another, deserve no attention. 

 The effects of different apertures may go some length ; but we 

 can perceive their particular application only in the few cases of 

 a simple separation. In most others there appears to be a 

 change of mixture ; but we perceive neither the precise changes 

 that are made, nor the cause of them. 



CCLXXXI. Till we can discover these more clearly, we 

 may in the mean time observe, that the actions of the vessels of 

 the secretory organ have a considerable share in determining 

 both the quantity and quality of the secreted fluid, and that 

 both very often are very little affected by the general state of 

 the circulation, or by the different conditions of the mass of 

 blood. 



CCLXXXII. It would seem that no other secretion but 

 those of perspiration and sweat are manifestly increased by the 



