328 RESPIRATION. 



logical relations, whose influence upon the character and motion of 

 the blood, the frequency of the respiration, &c., have not yet been 

 sufficiently investigated. We are, therefore, compelled, in endea- 

 vouring to deduce the laws of this interchange of gases, to trace 

 the modifications to which they are liable, to the remote as well as 

 the proximate causes from which they emanate. It is clear from 

 the above remarks, that the proximate causes of those alterations 

 which we meet with in the quantitative relations of the products of 

 respiration are solely based upon the prevailing physical and 

 chemical conditions ; the more remote causes, those, namely, of a 

 physiological character, can only influence this interchange of gases 

 inasmuch as they modify those essentially physical and chemical 

 relations. We cannot, therefore, hope to explain the influence 

 exerted by any definite physiological relation on the products of 

 respiration, until we have more clearly established its connection 

 with the physical and chemical fundamental conditions of respira- 

 tion. For this reason we shall in our further considerations of 

 the process of respiration, at once enter upon the influences which 

 purely chemical and physical relations exert on the interchange of 

 gases, in order thus to elucidate the influence of the physiological 

 conditions. Many difficulties here present themselves, one of the 

 chief of which is the imperfect knowledge we possess of the consti- 

 tution of the blood in ordinary cases, and the mere conjectural 

 nature of our knowlege of the amount of gases which it contains, 

 although as we have already observed, this constitutes one of the 

 two main factors of this process, and hence we are compelled to 

 enter more fully into these physiological conditions than we 

 should otherwise have done. 



We must, however, devote a few brief remarks to the general 

 results of the chemical investigation of the substances which meet 

 together in the act of respiration, before we enter more fully into 

 the causal connection of the modifications of the interchange of 

 gases. 



There is scarcely any portion of physiological chemistry which, 

 notwithstanding the great difficulties that oppose its investigation, 

 has been so circumstantially arid exactly elucidated as the respira- 

 tion. It would appear from the works of the older chemists, as 

 Lavoisier and Seguin,* Humphrey Davy,f Allen and Pepys^J 



* Md moires de 1'Acad. de Paris. 



t Researches, chemical and philosophical, chiefly concerning nitrous oxide, 

 or dephlogisticated air and its respiration. London, 1800. 

 Philosophical Transactions for 1 808. 



