322 



HISTOLOGY 



connective-tissue membrane at all points, but only with a comparatively 

 small surface of its body, while the rest of its cytoplasm broadens out into 

 a flange that reaches away from the pillar-like supporting mass and con- 

 nects with the flanges of other cells. 

 Thus an arch is formed between the 

 two cells, and, as the cells are arranged 

 in groups of three, all of whose points 

 of contact with the membrane are con- 

 tiguous, there is formed thereby a 

 network of channels or spaces running 

 everywhere between the outer cell- 

 flanges and the connective tissue. 

 These channels are occupied by the 

 network of capillaries carrying the 

 blood which is to exchange gases with 

 the air-medium in the lung. 



The capillaries have, a single-lay- 

 ered wall of endothelial cells, and this 

 wall is closely applied to the flanges 

 of the epithelial cells distad, to the 

 connective-tissue base proximad and 

 to the attached bodies of the respira- 

 tory cells laterad on all sides. 



In the section (Fig. 287) it can be 

 seen that the large, round nuclei of 

 the respiratory cells are nearly always 

 in the supporting cell mass that rests 

 on the connective tissue, while the 

 elongated (disk-shaped in surface view) 

 sections of the nuclei of the endothe- 

 lial cells that form the walls of the 

 blood capillaries are to be found at 

 any point of the circumference of the 

 FIG. 287. Part of a section of the wall of section of the capillary. 

 rt"c^cvr t t7cf:i,/:Se This arrangement with modified 

 of a respiratory ceil (the functional part detail holds for the vertebrate lung 



t^lSZbSlSZZZi to e****- Figure 2 88 shows a sur- 



face view of the respiratory epithe- 



in iii3.il. 



Another form of air-breathing, res- 

 piratory tissue is found among certain mollusks. Mollusks are 

 ordinarily water-breathers, and the water-breathing gill or cteni- 

 dium is a feature of much morphological importance. In a group 



resting on the connective tissue; ca.w., 

 walls of the blood capillaries; ca.nu., nu- 

 clei of capillaries; R C) blood cells. X435- 



