17 



had been stretched across the entrance to the sac. When this is the 

 case, the sac dilates suddenly beyond the opening. The sacs of the 

 lobulette are placed side by side, and are separated from each other 

 by thin membranous walls. Their shape, when properly inflated, 

 or when distended by some material which has set in them, as 

 gelatine, or a mixture of wax and turpentine, is polygonal. They 

 approach the circular form, but in consequence of their mutual 

 pressure, the parietes become somewhat flattened. The sacs increase 

 slightly in size as they pass from the bronchial tube to their fundus, 

 the latter being usually the broadest part of the sacs ; but they often 

 have an almost uniform diameter throughout. All the sacs pass 

 from the extremity of the bronchial tube towards the circumference 

 of the lobule of which the lobulette forms a part ; they consequently 

 radiate from the tip of the bronchial twig. The sacs connected with 

 one lobulette do not communicate with those of another lobulette. 

 As the sacs pass towards the boundary of the lobulette, they often 

 bifurcate ; and here and there circular orifices exist, leading to 

 smaller air-sacs, sometimes only to a small group of " air-cells," or 

 alveoli. If we trace the air-sacs from their fundus, we may say that, 

 passing from the periphery of the lobulette, and diminishing some- 

 what in size, they all terminate in the dilated extremity of the bron- 

 chial tube ; as they thus proceed they often join two or three 

 together, and these terminate in a single mouth. The tube which 

 results from the union of two or more sacs, is smaller in capacity 

 than the sacs taken together, but greater than either of them indi- 

 vidually. The dilated extremity of the bronchial tube above alluded 

 to constitutes the "point de reunion" of all the air-sacs, and may 

 be considered as the common centre of the lobulette. 



The walls of which the air-sacs are composed are exceedingly thin, 

 and much sacculated, t. e. they have in them a number of small, 

 shallow, cup-like depressions, separated from each other by portions 

 of membrane which are more or less raised and project into the 

 interior of the sac. The bottom of the air-sac presents the same 

 appearance as the lateral walls; and the cup-like depressions, or 

 alveoli, are there very numerous. The number of these alveoli varies 

 very much ; I have counted as many as ten at the fundus of an air- 

 sac in a cat's lung ; in the human lung I have counted five and six, 

 but the number is not usually quite so great. Close to the bottom 



VOL. x. o 



