beyond all doubt, that the vessels in these parts, 

 while they extend themselves, proceed in an un- 

 interrupted course, without communicating with 

 any others that no foreign humours which could 

 affect the blood, have access to them ; and that 

 consequently the blood is not exposed to the in- 

 fluence of any mixed chemical agency. But what 

 is it that here effects the chemical process, which, 

 from the very same particles of the blood, forms 

 those of saliva, milk, and urine? It cannot be 

 form and flexure of the vessels, since that can 

 only cause a greater or less delay ; and that this 

 alone cannot determine the formation of the se- 

 creted matter, common Chemistry will shew. 

 Consequently, there remains only the influence 

 of the nerves, which enter into these parts, and 

 which determine as well the nature of the se- 

 creted matter as its quantity ; but until our ex- 

 periments on unorganised matter shall have fur- 

 nished us with a chemical phenomenon, which 

 has any analogy with the operations of the nerves 

 on these occasions, \ve shall never be able to dis- 

 cover the laws of those operations, nor explain 

 the intimate nature of these processes. And if the 

 knowledge of the transformation of the blood 

 into other humours, which knowledge does in 

 itself bear an analogy to chemical phenomena 



