AND ANIMAL LIFE. 3 



the parietes of the thorax, and the expulsion of 

 the deteriorated air. 



IV. The object I have in view in the present 

 chapter, is to shew, that these two acts of respi- 

 ration have each an independent function, which 

 is continually regulating the distribution of the 

 blood throughout the system, and consequently 

 producing considerable modifications in its cir- 

 culation in the lungs. The principles, if found 

 satisfactory, will explain, in a beautiful and con- 

 sistent manner, a variety of phenomena intimately 

 related to organic and animal life. 



V. Physiologists have patiently investigated 

 the changes which the air experiences during 

 respiration, the conditions of arterial and venous 

 blood, and the influence of internal and external 

 means in diminishing or increasing the genera- 

 tion of animal heat, but have neglected to ascer- 

 tain in what way this principle is modified by 

 the different proportions of blood existing at diffe- 

 rent times within the chest. 



VI. This omission is somewhat extraordinary, 

 but it is principally to be referred to two causes ; 

 first, the belief that the circulation of the san- 

 guineous fluid is continuous and equable; and, 

 in the second place, to the nature or direction of 

 the experiments, the greater part of which was 

 confined to the lower animals. That the blood 

 is at all times in motion in every part of the 

 body, cannot be called in question ; but it is far 



