148 



ON THE ELEMENTARY PARTS OF THE HUMAN FABRIC. 



these structures, appreciable to ordinary sense as well as the innumerable 

 gradations by which they everywhere blend insensibly with one another, 

 are solely due to various degrees and kinds of modification wrought in the 

 form, quantity, and properties of these respective elementary parts." The 

 Mucous Membrane may be said, like the Serous, to consist of three chief 

 parts, the epithelium or epidermis covering its free surface, the subjacent 

 basement-membrane, and the areolar tissue, with its vessels, nerves, &c., 

 which forms the thickness of the membrane, and connects it to the adjacent 

 parts. Of the Epithelium and Epidermis, a general description has been 

 given in the preceding Section. The Basement- Membrane may be fre- 

 quently demonstrated with very little trouble, in the tubuli of the glands, 

 especially the kidney ; which are but very slightly adherent, by their exter- 

 nal surface, to the surrounding tissue. Its existence on the Skin, and on 

 many parts of the proper Mucous Membrane, has not yet been fully proved ; 

 but there can be no reasonable doubt of its continuity in these situations. 

 These two elements may be regarded as the essential constituents of Mucous 

 membrane ; which is thus found to be, strictly speaking, extra-vascular. Its 

 difference from Serous Membrane must be considered, therefore, as depend- 

 ing rather upon its arrangement, and upon the peculiar secretion of its epithe- 

 lium-cells, than upon any decided anatomical character. 



177. The tissues appended to these elements, and less essential to the 

 character of Mucous Membrane, are Capillary Blood-vessels, Absorbents, 



Fis. 43. 



Distribution of Capillaries at the sur- 

 face of the skin of the finger. 



Distribution of Capillaries in the 

 Villi of the Intestine. 



Nerves, and Areolar tissue. The former are almost everywhere abundant; 

 in the Skin they seem chiefly destined to supply the nervous papilla 3 , and 

 thus minister to its acute sensibility ; whilst in the Mucous Membrane 

 of the Alimentary canal, they seem more concerned in the functions of Ab- 

 sorption and Secretion ; and in the Glandular organs, they supply the mate- 

 rials for the last-named process. The Absorb- 

 ents are most abundant, as Lymphatics, in the 

 Skin ; and as Lacteals, in the Mucous Mem- 

 brane of the first part of the Intestinal canal; 

 but the Lymphatics are also largely distributed 

 through some of the Glandular organs. The 

 Skin is the only part of this system, which is 

 largely supplied with Nerves ; except the Con- 

 junctival Membrane, and the Mucous Mem- 

 brane of the Nose: hence the sensibility of this 

 structure is usually low, although its import- 

 ance in the organic functions is so great. The 

 Areolar tissue of Mucous Membranes usually 

 makes up the greatest part of their thickness ; and is so distinct from the sub- 

 jacent layers, as to be readily separable from them. It differs not, however, 



Distribution of Capillaries around 

 follicles of Mucous Membrane. 



