in relation to Disease. 41 



changes of weight, is a curious subject of inquiry in many points 

 of view. Tt is chiefly and most familiarly noted, in disturbances 

 of the balance of circulation throughout the body ; and parti- 

 cularly in that of the head and lungs ; from obvious causes as 

 respects the economy of these organs. The functions of the 

 lungs, indeed, are subject to this influence in several ways ; even 

 the mechanical part of the process being in sone part concern- 

 ed ; as well as the balance between the external air and that 

 within the bronchial cells ; and the relation of the whole to the 

 quantity of blood in the pulmonary circulation. While the ac- 

 tion of the heart must necessarily be affected by all which thus 

 tends to disturb the equal movements of the circulating fluid. 



Another consideration again regards the relative effect of air 

 of different density, in producing the proper changes on the 

 blood. It is clear that there exists a point of rarefaction, at 

 which this quantity of oxygen is insufficient for the purposes 

 it has to fulfil. Or, giving the statement its most general form, 

 there must be a particular specific gravity of air (concurring 

 probably with the medium barometrical pressure), which is best 

 fitted for the necessities of the function ; and all deviations 

 from which, in one degree or other, interfere with the complete- 

 ness of its performance. 



These eflfects, however, under ordinary circumstances, and 

 in healthy state of body, are slight or inappreciable in amount ; 

 limited by the range of barometrical variation, and by the 

 usual slowness of the changes taking place. They are aug- 

 mented of course when the variation is more rapid and of 

 greater extent ; — still, however, depending on changes in the 

 state of respiration ; and on irregular distribution of blood, 

 from the altered balance of pressure between the external and 

 internal parts of the body. The latter cause might be ex- 

 pected to affect most the vascular system ; seeing its structure, 

 functions, and the mechanical principles which in part deter- 

 mine the motions of fluids, even in the vessels of the living 

 body. The common observations with the air-pump and cup- 

 ping-glasses shew the facility with which these vascular tex- 

 tures, and the contained fluids, yield to any such change of 

 balance; and in the effects in the diving bell, on the head 

 more especially, produced by an increase of only one-fifth, or 



