134 DR. E. KLEIN. 



therefore, led to conclude that these small cubical cells are 

 capable of expansion and contraction. In the first instance they 

 are transparent, large and flat cells, in the second they are less 

 transparent, more granular, smaller and more cubical. During 

 deep inspiration the increase of volume of the air vesicles ne- 

 cessitates an increase of surface of the lining epithelium, and 

 this takes place partly at the expense of those small cells, whereas 

 during a deep expiration the reduction in surface produces again 

 a reduction of some of the large flat cells into small cubical 

 elements. Thus we have precisely the same condition as that 

 in the endothelium of the surface of the pulmonary pleura. 

 But I find also a similar relation to exist with regard to the 

 epithelium of the smallest bronchi. 



If in a section of lung I compare a minute bronchus, which — 

 judging by the thinness of its muscular coat and by the large 

 size of its lumen — is to be regarded as being in a distended con- 

 dition, with one that — judging from the opposite appearances, 

 viz. thickness of muscular coat and smallness of lumen — is in 

 a contracted state, I notice a distinct change in the thickness of 

 the epithelium. In the one case the epithelium is composed of 

 a single layer of short columnar cells ; in the other the cells are 

 very elongated, and even so placed as to resemble laminated cells. 

 A similar difi'erence has been pointed out by Arnold^ with regard 

 to the epithelium covering the mucous membrane of the tongue 

 of frog, viz. that the epithelium is composed of a single layer 

 of cells (grooves between papillse), but appears laminated where 

 the cells are more pushed together (papillse themselves). 



With reference to the bronchi, I am quite aware that it may 

 be urged that except by comparing two bronchi of exactly the 

 same order no definite conclusion can be arrived at as regards 

 the relative thickness of the different parts; and I do not 

 say that in arriving at the above conclusion I have made absolute 

 measurements. But it must be admitted that even without such 

 it is perfectly possible to say in a given preparation whether two 

 bronchi are approximately belonging to the same order, and which 

 of them is contracted and which not, I presume any one who 

 has some experience in the examination of microscopic sections 

 will be capable of determining approximately whether, e.g. two 

 arteries seen in transverse section, but of which one is contracted 

 the other distended, are approximately of the same order. 



Prom all I have seen it seems probable that passing from the 

 maximum of a deep inspiration to that of a deep expiration the 

 epithelial cells lining a minute bronchus change from the elon- 

 gated shape into that of a short columnar cell. 



' Vircliow's ' Archiv,' Band Ixiv, p, 203, and passim. 



