ORGANS OF RESPIRATION. 383 



uncertainty which seems to me to be attached to this test, 

 I yet consider that by a careful examination of the con- 

 dition of the blood vessels in the two lungs, the question 

 may in most cases be satisfactorily determined, whether has 

 the lung been artificially or naturally extended with air ? 



Previous to birth, it is known that the circulation of blood 

 through the lungs is of a very limited character, and that it 

 is only after that period, and during the first act or acts of 

 respiration that the great mass of vessels, principally capil- 

 laries, become carriers of blood. 



The vessels then, in the one case, viz. before respiration, 

 will be almost devoid of blood, and, in the other, after that 

 act, replete with that fluid. 



Dependent upon this difference in the condition of the 

 vessels, we shall notice certain distinctive features, both 

 general and microscopical, in the case of the artificially and 

 the naturally inflated lung. The former, after inflation, will 

 be observed to collapse to nearly its previous size on the 

 escape of the air ; it will present a pale appearance, especially 

 evident if the lung be cut into, and when examined with a 

 lens, it will be seen that the interspaces between the lobules 

 and cells are pale, not being occupied with red vessels. Now, 

 on the other hand, the latter will be characterized by appear- 

 ances the very reverse : it, after inflation, will not collapse to 

 nearly its previous size ; it will be somewhat red, and the in- 

 tervals between the lobules and cells will be seen to contain 

 red vessels, in which, as well as in the capillaries, the higher 

 powers of the microscope will reveal the existence of red 

 blood discs. 



PATHOLOGY* 



Now that the normal anatomy of the lungs is well under- 

 stood, the several pathological conditions of those organs 

 admit of a precise and satisfactory explanation. The prin- 

 cipal diseases of the lungs, upon the exact nature and seat of 

 which the microscope affords, directly or indirectly, satisfac- 



H II 3 



