380 THE SOLIDS. 



of fibro-elastic tissue present in the lungs. (See Plates 

 XL VII. and XLVIIL) 



That the air cells do really communicate freely with each 

 other (a fact which is now generally admitted, but which is 

 yet denied by Dr. T. Williatfis * ) may be satisfactorily deter- 

 mined in many ways. Thus it is by no means difficult to see 

 the circular and ever patent openings of communication, both 

 in injected and uninjected lungs ; again, by injection with size, 

 perfect casts of the cells may be obtained : these casts when 

 a fragment of lung is torn up in water, readily escape, by 

 which means the variety presented by the cells in form and 

 magnitude may be accurately determined, as well as the shape 

 and number of the openings of communication. (See Plate 

 XLVIII. jfy. 2.) Not unfrequently these casts are more or 

 less coated with the epithelium lining the cells, as also repre- 

 sented in the figure just referred to. 



So perfect and consistent are the models of the air cells 

 procured in this way, that for some time I was led to enter- 

 tain the idea that each little mass of size was really enclosed 

 in a delicate and structureless membrane, which I conceived 

 to be the true air cell lining, and I have scarcely yet been 

 able to discard the notion altogether from my mind. If the 

 lung be injected with tallow, we do not procure casts in the 

 same number and perfection, because the tallow adheres 

 closely to the sides of the air cells. Sometimes I have seen 

 casts even without injection ; but the occurrence of these is 

 rare. It is, therefore, yet possible that I have not been 

 deceived, and that a structureless membrane does really line 

 the air cells, a modification of the mucous lining of the 

 tubes, the " basement membrane " of Bowman. 



It would appear from an examination of these casts, that 



the number of openings of communication varies from one 



to five ; usually, however, but one, two, or three are present. 



When looking attentively at these casts, covered with 



' * Essay on the Structure and Functions of the Lungs. College of 

 Surgeons, London, 1842. 



