MUCOUS MEMBRANES. 131 



words will be necessary regarding the mucous membranes, which resem- 

 ble the skin so much in their properties, as to be, with propriety, termed 

 dermoid. If we trace the skin into the various outlets, we find, that a 

 continuous, soft, velvety membrane epithelium exists through their 

 whole extent; and, if the channel has two outlets, as in the alimentary 

 canal, this membrane, at each outlet, commingles with the skin; and 

 appears to differ but slightly from it. So much, indeed, do they seem 

 to form part of the same organ, that physiologists have described- the 

 absorption, that takes place from the intestinal mucous membrane, as 

 external. They cannot, however, in the higher order of animals, be 

 considered completely identical ; nor is the same membrane alike in its 

 whole extent. They have all been referred to two great surfaces ; the 

 g astro-pulmonary comprising the membranes of the outer surface of 

 the eye, ductus ad nasum, nose, mouth, and respiratory and digestive 

 passages ; and the genito-urinary which line the whole of the genital 

 and urinary apparatuses. In addition to these, a membrane of similar 

 character lines the meatus auditorius externus, and the excretory ducts 

 of the mammae. 



The analogy between the skin and mucous membranes is farther 

 shown by the fact, that if we invert the polypus, the mucous membrane 

 gradually assumes the characters of skin; and the same circumstance 

 is observed in habitual descents of the rectum and uterus. 



In the mucous membranes especially at their extremities, which 

 appear to be alone concerned in the sense of touch the same super- 

 position of strata is generally considered to exist as in the skin viz., 

 epidermis or epithelium, rete mucosum, corpus papillare, and cutis 

 vera. They have, likewise, similar follicles, called mucous; but nothing 



A Fig. 47. B 



Separated Epithelium Cells from Pavement-Epithelium of the Mucous 



mucous membrane of mouth. Membrane of the smaller bronchial tubes. 



b. With nuclei, c. And nucleoli. a. Nuclei with double nucleoli. 



analogous to the hairs ; unless we regard the teeth to be so, in corre- 

 spondence with the views of Meckel, De Blainville, and others. 



The attention of anatomists has been closely directed to the ultimate 

 structure of the mucous system. In the mucous tissues two structures 

 have been separately described, especially by Mr. Bowman, 1 who has 

 thrown much light on the subject. These are the basement membrane 

 as he terms it and the epithelium. The former is a simple, homoge- 

 neous expansion, transparent, colourless, and of extreme tenuity, situate 



1 Cyclopaedia of Anat. and Physiology, pt.xxiii. p. 486, April, 1842. 



