3 86 



STRUCTURE OF THE AIR-VESICLES. [sect* 177. 



Fig. 1G0. 



at first, numerous racemose glands, which, however, disappear upon 

 canals of 1" to 1 V" in diameter, although Remak says he has seen 

 them in the walls of the finest bronchia just before they enter the 

 ultimate lobules. 



Pulmonary Vesicles. — In these we may demonstrate the existence 

 of two layers, the one fibrous, the other epithelial ; and I am not 

 prepared to admit that there are any other coats than these. The 

 fibrous coat is obviously continued from the bronchia, and consists 

 of the mucous membrane, which has become very thin and con- 

 joined with the fibrous layer. This coat is completely destitute of 



smooth muscular fibres, and 

 consists of a homogeneous 

 stratum of connective tissue, 

 together with elastic fibres 

 and numerous vessels. The 

 elastic fibres {p'ooo$'" to 

 0002'") appear chiefly in 

 the form of separate trabe- 

 cular and stripes, coursing 

 upon the borders of air- 

 cells and around their open- 

 ings ; they anastomose in 

 all directions with each 

 other, and they form a firm 

 framework, between which 

 the softer vascular parts of the alveoli are stretched. These elastic 

 trabecular are generally composed of yellow fibres, arranged as 

 closely as possible, the meshes of the network appearing only as 

 very narrow fissures, so that it is difficult to recognise the nature 

 of the tissue we are observing; but sometimes the fibres are more 

 loosely connected, and then their elements are distinct. The 

 trabecular meet and coalesce with each other around the pulmonary 

 vesicles, so that the boundaries of the separate air-cells are rarely 

 to be distinguished. A few elastic fibres of the finer kind also 

 proceed from the trabecular into the remaining walls of the pul- 

 monary vesicles, and become connected therein to form a wide 

 network. The connective tissue of the air-cells, which appears per- 

 fectly homogeneous, gives place to the elastic elements and vessels, 

 where these are in large quantity, and comes to view, so to speak, 

 only in the walls of the alveoli, between the elastic trabecular, 

 serving to connect the numerous capillaries. 



The epithelium of the pulmonary vesicles is an ordinary pave- 





A human pulmonary vesicle, with the parts ad- 

 joining; magnified 350 times, a. Epithelium. 6. 

 Elastic trabecule, e. More delicate walls between 

 the trabecular, with fine elastic fibres. 



