376 OF RESPIRATION. 



cavity ; and a rush of air takes place down the air-tubes, and into the 

 remotest air-cells, to equalize the density of the air they include (which 

 has been rarefied by the dilatation of the containing cavities) with that 

 of the surrounding atmosphere. 



677. The diameter of the ultimate air-cells of the Human lung 

 varies from about the l-200th to the l-70th of an inch. Their shape 

 is irregular, and their walls are, for the most part, flattened against 

 each other. Each of the ultimate ramifications of the bronchial tubes 

 communicates with a cluster of these air-cells grouped around it ; those 

 which are in immediate proximity with the tube open into it by well- 

 defined circular apertures, and the others communicate with it by open- 

 ing into these and into each other. Each air-cell is lined by an exten- 

 sion of the mucous membrane from the bronchial tubes ; but this does 

 not eem to be furnished with an epithelial covering. Between the 

 adjacent air-cells, is a network of fibrous tissue, that forms the connect- 

 ing medium by which they are held together ; this tissue appears to be 

 of the elastic kind. The pulmonary arteries subdivide into branches, 

 whose ultimate ramifications form an extremely minute capillary plexus ; 

 and this is disposed between the walls of the adjacent air-cells, so that 

 each portion of this plexus comes into relation with the air (through the 

 lining membrane of the contiguous air-cells) on loth sides, an arrange- 

 ment which is obviously the most favourable that can be to the aeration 

 of the contained blood. It has been calculated by M. Rochoux, that 

 the number of air-cells grouped around each terminal bronchus is little 

 less than 18,000 ; and that the total number in the lungs amounts to 

 six hundred millions. If this estimate be even a remote approximation 

 to the truth, it is evident that the amount of surface exposed by the 

 walls of these minute cavities, must be very many times greater than 

 that of the whole exterior of the body. 



Fig. 106. 



Arrangement of the Capillaries of the air-cells of the Human Lung. 



678. The larger bronchial tubes are more or less cartilaginous ; but the 

 smaller branches do not possess any such deposit in their walls, though 

 still retaining their circular form. We find in the latter a fibrous 

 structure, which seems to possess the properties of non-striated muscle ; 

 and by this, the diameter of these tubes appears to be governed. The 



