MUCUS. 123 



Mucous membranes, therefore, as might be inferred from 

 the concluding passage of the preceding paragraph, do not all 

 present a constitution precisely similar the one to the other ; 

 and on their differences of organization a division of them 

 into three classes may be instituted, as has been pointed out 

 by M. Donne. 



The first class of mucous membranes comprises those which 

 are contiguous to the outlets of the body, and which are to be 

 regarded as extensions of the skin, participating in all its 

 properties : thus the fluid secreted by this class of mucous 

 membranes manifests, like that of the skin, an acid re-action, 

 and the same epithelium which invests the latter belongs also 

 to the former ; in other respects the correspondence is like- 

 wise exhibited, the membranes under consideration manifest 

 the same sensibility, the same freedom from hemorrhage, 

 which characterise the skin ; they in like manner ulcerate less 

 readily, and are never furnished with the vibratile cilia3 

 which belong to the second class of mucous membranes, viz. 

 the true. This first described class of membranes may be 

 denominated false mucous membranes; and as an example of 

 it the vagina may be cited. 



The membranes which belong to the second class are 

 situated more internally than the last, and have scarcely any 

 thing in common with those of the first class: the mucus 

 secreted by them constantly exhibits an alkaline re-action, 

 and the epithelium which invests them is of a totally dif- 

 ferent structure, the cells which constitute it being cuneiform, 

 and in some situations provided with numerous vibratile 

 cilia? : the general properties of this class of membranes are 

 also opposed to those of the previous division : thus they are 

 but little sensitive to the touch, are frequently the seat of 

 hemorrhages, and ulcerate with much facility. The mem- 

 branes of this class are to be considered as the true mucous 

 membranes, and that which lines the trachea and bronchi 

 may be instanced as the type of this class. 



The third class is more artificial than the two preceding ; 

 the membranes which it comprises exhibit in a greater or 

 less degree the characters of each of the divisions already 



