38 
thousandth part of an inch in diameter ; whilst the pulmonary 
vesicle itself is twenty times greater in diameter than that of one of 
the capillaries which are distributed on its walls. Between the 
atmospheric air in the cells, and the blood in those minute vessels, 
nothing intervenes but the thin membranes of the cells and capil- 
laries and the delicate epithelial lining of the former; and the 
exposure of the blood to the air is the more complete, because the 
folds of membrane between contiguous cells, and often the spaces 
between the walls of the same, contain only a single layer of capil- 
laries, both sides of which are thus at once exposed to the air. To 
summarise, in conclusion :—It has been shewn that, as regards the 
minute structure’ of the human lung, each terminal bronchial tube 
is connected with an entire group of air-cells, not ending in a single 
air-cell, nor the tubes themselves anastomosing with each other ; 
also that the ciliated epithelium of the bronchial tubes does not 
extend to the air-cells, but that each air-cell has a single layer of 
pavement epithelium composed ef minute polygonal cells. Such, 
then, is the respiratory apparatus in man—complicated, and yet, 
when unravelled by the aid of the microscope, and studied in the 
light of comparative anatomy and embryology, beautifully simple in 
structure. In its multitudinous air-sacs and air-cells, it affords a 
very large amount of surface for the aeration of the blood in a com- 
paratively small space ; and so important is this apparatus in its 
relation to the well-being of the system generally, that the smallest 
alteration of its structure leads to the most serious functional 
disturbances, and eventually, if unchecked, to formidable disease 
and death. 
Dr. Anderson’s paper was profusely illustrated by diagrams. 
Mr. Henry Asupy made a few observations respecting the 
similarity that existed between the brain of man and that of 
the lower animals; and said that, as had been shown by Dr. 
Anderson in his introductory remarks, the gradual complication 
of the structure of the lung in the lower animals had kept pace 
with the development of the same organs in the human embryo ; 
and this probably had led to the evolution theory propounded 
by Darwin in his famous work. we 
A cordial vote of thanks to Dr. Anderson was accorded by the 
President, on behalf of the Club. : 
The President and nine other Members placed their microscopes 
at the disposal of Dr. Anderson to illustrate his paper. 
