STRUCTURE OF THE LUNGS. 523 



lines in the interstices of the lobuli, to be hereafter mentioned. 

 It is also diffused in small quantities throughout the substance 

 of the lungs. 



The sources of this substance, and the use of it, are unknown. 



The lungs are of a soft spongy texture ; and, in animals that 

 have breathed, they have always a considerable quantity of 

 air in them. 



They consist of cells, which communicate with the branches 

 of the trachea that ramify through them in every part. These 

 cells are extremely small, and the membranes which compose 

 them are so thin and delicate, that if they are all filled by an 

 injection of wax, thrown into the trachea, the whole cellular 

 part of the lung will appear like a mass of wax. If a corroded 

 preparation be made of a lung injected in this manner with 

 force, the wax will appear like a concretion. 



These effects of injections prove that the membranes of 

 which the cells are formed are very thin ; and, of course, that 

 their volume is very small when compared with the capacity 

 of the cells. 



In those corroded preparations, in which the ramifications of 

 the bronchia are detached from the wax of the cells, these 

 ramifications become extremely small indeed. The cells above 

 alluded to are in fact but the ultimate termination of the last 

 branches of the bronchia in small dilated sacs, called the bron- 

 chial or pulmonary cells. 



If the lungs of the human subject, or of animals of similar 

 construction, be examined when they are inflated, their cellu- 

 lar structure will be very obvious, although their cells are so 

 small that they cannot commonly be distinguished by the naked 

 eye. Each of the extreme ramifications of the bronchia 

 appears to be surrounded by a portion of this cellular sub- 

 stance, which is gradually distended when air is blown into 

 the ramification. 



This cellular substance is formed into small portions of 

 various angular figures, which are denominated Lobuli : these 

 can be separated to a considerable extent from each other. 



