THE CAPILLARIES. 225 



and separating from each other, and yet unlike either the arte- 

 ries or veins, retaining always the same diameter. In the lungs 

 he considers this system particularly well developed ; and that 

 the walls of the ultimate branches of the arteries and vejns are 

 abruptly terminated, and appear cribriform under the microscope, 

 when they communicate with or divide into the capillary arteries 

 and veins. 



Fig. 174. 



The delicate capillary vessels of the lungs are inclosed in 

 the mucous membrane, forming the bronchial cells. The 

 mucous membrane of these cells being derived from the 

 bronchia, is of course continuous throughout the lungs, and 

 may be regarded as a delicate membrane filled with a net-work 

 of capillary vessels, formed into cells for the mere purpose of 

 allowing an extensive surface to be compressed into a small 

 space : upon one surface of this membrane, the pulmonary 

 vessels maintain a stream of blood, divided into currents of the 

 minutest size ; and on the other, air is admitted through the 

 bronchia, the oxygen of which acts upon the particles of blood 

 in their most divided state. 



The pulmonary or lesser circulation, (which is conducted 

 through the pulmonary arteries and veins,) would be perfectly 

 isolated from the greater or systemic, (that of the aorta and vena 



