THE RESPIRATORY ORGANS AIR-CELLS. 579 



membrane and fibrous tunic of the bronchial tubes (Kolliker) ; but 

 it is, in fact, made up of elastic tissue and vessels, in a homogeneous 

 matrix. The elastic fibres present the form, chiefly, of separate 

 trabeculse and filaments, running between the epithelial linings of 

 the air-cells, and supporting the capillary vessels. (Fig. 405.) By 

 anastomosing with each other, they constitute a firm frame, on 

 which the softer vessels are stretched, while over them the epithe- 

 lium is laid. These elastic trabeculse mutually coalesce, so that, for 

 the most part, the boundaries of the separate air-cells cannot be 

 recognized where the latter abut upon each other. 



The epithelium of the air-cells is the simple scaly variety, com- 

 posed of pale, polygonal, granular cells, averaging %-QQ-Q of an inch 

 in width, and 3^0 f an i ficn m thickness. It lies immediately on 

 the fibrous walls (just described) of the air-cells. 



C. The capillary plexus of the lobules is one of the closest in the 

 human body, presenting rounded or oval meshes g^^ to J^-Q of 



Fig. 406. 



Fig. 406. Arrangement of the capillaries of the air-cells of the human lung. 



Fig. 407. Slightly oblique section through a bronchial tube. a. The cavity of the tube. b. Its 

 lining membrane, containing bloodvessels with large areolse. c, c. Perforations in this membrane, 

 where it ceases, at the orifice of the lobular passages, d, d. e, e. Spaces between contiguous lobules, 

 containing the terminal pulmonary arteries and veins supplying the capillary plexus (/,/), to the 

 meshes of which the air gains access by the lobular passages. 



an inch wide, and vessels :joW ^ 2?u<j ^ an i ncn m diameter. 1 It 

 lies in the wall of the air-cells, about T2 i^ of an inch from the 



1 Todd and Bowman make the capillaries so large (y^Vfi of an inch) as to secure 

 a free circulation, and intimate that the blood traversing them moves If inch per 

 minute. Each capillary extends over 8 to 10 air-cells, and the air remains in con- 

 L act with the blood l.J second. (Rainey.) 



