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in some of the air-sacs in the human lung, a circular opening, similar 

 to those already alluded to as leading to other sacs, small and con- 

 stricted, is often seen, and has the appearance as if it led to another 

 sac ; on examination, however, it will be found to be produced by 

 a projection inwards of the membrane of the sac, and to lead to 

 a small cavity, or group of alveoli. 



The number of alveoli existing in the air-sacs varies Rossignol 

 states that each "infundibulum" (air-sac) contains from ten to twenty 

 alveoli. My own observations entirely accord with this statement. 

 I have found the number varying from eight to twenty. 



The air-sacs externally, by their fundus, rest on the pleura, but 

 within the substance of the lung they in part rest on, and are sup- 

 ported by, the bronchial tubes and blood-vessels. 



The air-sacs are separated from each other by thin walls, the 

 membrane composing which, in a lung inflated and dried, is very 

 transparent. The projection of this membrane in the shape of a 

 thin process, having a sharp margin, constitutes the septa between 

 the alveoli ; and wherever an opening exists leading into a smaller 

 sac, this membrane projects in a similar way, and forms a circular 

 orifice which is much smaller than the cavity to which it leads : 

 the sac, in fact, dilates abruptly on the distal side of the opening. 

 It is in the membrane composing these walls, and in the septa of the 

 alveoli, that the capillaries of the pulmonary artery are spread out. 



The number of air-sacs belonging to a lobulette varies : I have 

 counted as many as six communicating with a bronchial tube incised 

 horizontally, so that probably only half the sacs were left ; this, 

 however, is a larger number than is usually found ; from six to eight 

 or ten is the more common number. 



Each lobulette is separated from those by which it is surrounded, 

 by walls which appear to resemble in every way the walls of the air- 

 sacs ; and in an adult inflated and dried lung, careful observation is 

 necessary to make out the partitions. That perfect septa do exist, 

 is proved by laying open, in a recent lung, a bronchial tube to its 

 ultimate division, when by placing a fine blowpipe in it, and blow- 

 ing down it, a single lobulette is alone inflated. 



The separation of the lobulettes is further distinctly perceptible in 

 the recent lungs of infants, in which the line of demarcation between 

 the lobulettes is often plainly seen, on the surface. 



