XII MUCOUS MEMBRANE 545 



millimeter, called a micro-millimeter, and indicated by the 

 symbol /ii, of which all greater magnitudes are multiples.^ 

 Thus, if the extreme point of a rabbit's hair has a 

 diameter of 1m, the middle of the shaft will be 2o/i, and 

 the shaft of a human head hair 80/i. 



Adopting this system, the deep cells of epidermis have 

 on an average a diameter of 12/x or more, the nuclei of 

 4/x to 5/i, while the superficial cells are plates of about 2bn, 

 the nuclei retaining about the same dimensions. The 

 diameter of a white corpuscle of the blood is about lOu, 

 that of a red corpuscle being 7fi to 8/x. Hence the deep 

 cells of the epidermis are rather larger than white blood 

 corpuscles, and the uppermost ones much larger, at least 

 in superficial area. 



9. The Epithelium of Mucous Membrane.— The 

 mucous membrane lining the alimentary canal, as has 

 been stated, is framed on the plan of the skin, inasmuch 

 as it consists of a vascular dermis, and a non-vascular 

 epithelium, the latter being composed of cells in juxta- 

 position. But except in the region of the mouth, where, 

 as we have seen, the epithelium, like the epidermis, is 

 composed of many layers of cells, arranged as a soft 

 Malpighian layer and a hard corneous layer, and the 

 oesophagus where the structure is similar, the epithelium 

 of the alimentary canal and the continuations of that 

 epithelium into the ducts and alveoli of the various glands, 

 consists of hardly more than a single layer of cells placed 

 side by side. Hence in a vertical section of the mucous 

 membrane the vascular part is seen to be covered by a 

 single row of soft nucleated cells ; though sometimes a 

 second row of inconspicuous small cells may be seen 

 below the latter. The cells constituting this single layer 

 vary in shape, being cylindrical or conical or, as especially 

 in the glands, cubical or spheroidal ; but they always are 

 delicate masses of protoplasm, each containing a nucleus. 



1 Since 1 millimeter is very nearly equal to ^ of an inch, n = rdva ©^ 

 an inch. 



N N 



