192 PHYSIOLOGY. 



The mechanical powers of pressure, commonly spoken of, do 

 not in fact take place, nor are their supposed effects any ways 

 consistent with sound philosophy ; and, on the other hand, it is 

 very probable, that the changes produced are the effects either 

 of chemical separation or mixture. 



What has been supposed to be performed in this way by an 

 absorption of air, or of a particular matter from it, is very un- 

 certain in fact, and has led to a still more uncertain reasoning. 



It is now certain, that a quantity of mephitic air, and perhaps 

 some other matters, are constantly exhaling from the lungs of 

 living animals, and are carried off by the atmospherical air al- 

 ternately entering and issuing from the lungs. This is a pretty 

 certain evidence that some change of mixture is going on in the 

 fluids passing through the lungs ; but from what particular por- 

 tion of the fluids the mephitic air proceeds, or what is the effect 

 of its separation, we know not ; and indeed, as we have said be- 

 fore, what are the effects of the action of the lungs upon the 

 state of the fluids, we are very uncertain. Upon the whole, we 

 still know but little of the production or formation of the animal 

 fluids ; and therefore, from the consideration of their formation, 

 we have learned little of their nature ; but we must now try to 

 discover what we can of it, by examining these fluids as they are 

 found already formed in the blood-vessels. 



CHAP. II. OF ANIMAL BLOOD. 



" From the earliest accounts of physic with which we are ac- 

 quainted, it appears, that from the most ancient times down to 

 the present, physicians have been attached almost entirely to 

 the study and consideration of the fluids ; and from the sup- 

 posed state of these, have endeavoured to explain the phenom- 

 ena both of health and of sickness. In this, however, they ap- 

 pear to me to have been unfortunate ; for, not to mention the 

 imperfection and falsehood of the many speculations, both of 

 Galenists and chemists, which have formerly prevailed on this 

 subject, I would venture to assert, that the doctrine concerning 

 the fluids is still the most imperfect part of our physiology. 

 Every thing systematical that has been delivered upon it % till 

 within these last forty years, may be totally disregarded ; for it 



