580 THE TISSUES. 



epithelium; some of the vessels also projecting fairly into the cells, 

 since they are sometimes thicker than the walls of the latter. It 

 continues not only over all the air-cells of the same lobule, but also 

 anastomoses with the plexuses of the contiguous lobules. A great 

 portion of the capillaries are also situated between, and in relation 

 with, two air-cells at the same time. Fig. 406 shows the capillary 

 plexus seen on the walls of, and between, the air-cells, after the 

 epithelium is removed. There is also a capillary plexus on the 

 surface of the smallest air-tubes, and even extending to the trachea, 

 in continuation with the preceding, characterized by the elongated 

 form of its meshes (Heale 1 ), and formed of vessels almost as fine as 

 those of the air-cells (-joVir to 24^ of an inch). (Fig. 407.) Only 

 the pure aerated blood enters this plexus, since it has previously 

 circulated through the capillaries of the air-cells. 



Function of the Respiratory Apparatus. 



The air-tubes merely conduct the air to and from the air-cells of 

 the lungs. It is not probable that the epithelium becomes desqua- 

 mated to any considerable extent in disease. At least, it is certain 

 that in croup an exudation may occur through the epithelium with- 

 out detaching its cells, and which may subsequently be coagulated 

 into a " false membrane," falsely so called, or undergo degeneration 

 into pus (pp. 497 and 189). 



The lungs are the aerating organs of the blood ; i. e. they secrete 

 carbonic acid gas from the blood in the capillary plexus in the walls 

 of the air-cells, and absorb oxygen at the same time into the blood. 

 The layers interposed between the air in the cells, and the capillary 

 vessels, are but ^Viy of an inch thick on an average. The whole 

 amount of surface presented to the air by the six hundred million 

 air-cells in the lungs, has been estimated at 132 square feet, or more 

 than eight times as great as the cutaneous surface of the body ; 8 and 

 all the blood in 4;he body traverses the capillary plexus spread out 

 on the air-cells, probably, within the space of two minutes. 



Development of the Lungs. 



The lungs appear a little after the liver, as two hollow protru- 

 sions of the anterior wall of the pharynx ; into the composition of 

 which the epithelium and the corium of the pharynx equally enter. 



1 American Medical Monthly, vol. ii. p. 302. 



2 Lindenau computed the whole surface of the air-cells and the air-tubes at 

 2642 square feet ! ! ! 



