structure of the cells of the human lungs, &c. 63 
so that it is only such blood that is decarbonised by the car- 
bonic acid that is carried off' by expiration. 
Respiration is necessary for carrying on the functions of 
life, by a supply of oxygen, and the removal of the excess of 
carbon ; but it is not required for the simple support of the 
principle of life when no action is going on. 
The garden snail excludes the atmospheric air from, the 
lungs, by means of an operculum formed of mucus, during 
the winter season ; and remains alive sealed up hermetically, 
till the warmth and the moisture of the spring dissolve the 
operculum, and expand a globule of air that had been shut 
up in the cavity of the lungs, the escape of which opens the 
communication with the outer air and restores respiration. 
EXPLANATION OF THE PLATES. 
Plate VIII. Shows the cells of the human lungs as they 
appear on the field of the microscope. 
Fig. 1. represents -g^th part of an inch of the external 
surface of the human lung, the cells of which are filled with 
quicksilver ; magnified 20 diameters. 
Fig. 2. a transverse section of —^th part of an inch of the 
human lung, in which the arteries are filled with red, and the 
veins with yellow minute injection ; magnified 20 diameters. 
Fig. 3. the transverse section of a single cell from fig. 2. 
with the parts immediately surrounding it; magnified 400 
diameters. 
Plate IX. Shows the structure of the human lungs imme- 
diately surrounding the air cells. 
Fig. 1. the interstitial substance between the cells, in a 
