FORMATION OF THE TISSUES MUCOUS MEMBRANES. 501 



Mr. Bowman has justly remarked,* " of certain elements which the Anatomist 

 may detect and discriminate ; some of them being essential, others appended 

 or superadded : and the broad characteristic distinctions between these struc- 

 tures, 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 Mem- 

 brane may be said, like the Serous, to consist of three chief parts, the epithe- 

 lium or epidermis covering its free surface, the subjacent basement-membrane, 

 and the areolar tissue, with its vessels, nerves, &c., which forms the thick- 

 ness of the membrane, and connects it to the adjacent parts. Of the Epithelium 

 and Epidermis, a general description has been already given ( 621, 622). 

 The cells of which they are composed, have very different offiOs in the dif- 

 ferent situations ; on the Skin, they serve only for protection ; on the proper 

 Mucous Membranes, they secrete a protective Mucus ; and in the Glandular 

 tubuli and follicles they elaborate the peculiar secretions of the respective 

 organs. The Basement- Membrane may be frequently demonstrated with very 

 little trouble, in the tubuli of the glands, especially the kidney; which are but 

 very slightly adherent, by their external 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 con- 

 sidered, therefore, as depending rather upon its arrangement, and upon the 

 peculiar secretion of its epithelium-cells, than upon any decided anatomical 

 character. All the Mucous Membranes are exposed to the contact of irritating 

 substances, or of air ; and the secretion which they form seems to have for its 

 special object, to protect them from these sources of injury. The constant 

 elaboration of it requires a continual renewal of its Epithelium-cells ; and 

 hence, whilst those of Serous Membranes seem to have considerable perma- 

 nence, those of Mucous Membranes are being continually thrown off and 

 reformed. Where this secretion is required in great abundance, we find that 

 the surface of the membrane is greatly extended, by being involuted into folli- 

 cles, which are more or less closely set together ; these follicles are lined by a 

 layer of epithelium-cells, continuous with that of the free surface ; and their 

 secretion is destined for the same purpose. In some parts of the intestinal 

 tube, the follicles are very long, and are closely set together ( 704, Fig. 170). 

 641. The tissues appended to these elements, and less essential to the cha- 

 racter of Mucous Membrane, are Capillary Blood-vessels, Absorbents, Nerves, 

 and areolar tissue. The former are almost everywhere abundant; in the 

 Skin they seem chiefly destined to supply the nervous papillae, and thus 

 minister to its acute sensibility ( 117) ; whilst in the Mucous Membrane of the 

 Alimentary canal, they seem more concerned in the functions of Absorption 

 ( 463) and Secretion ; and in the Glandular organs, they supply the mate- 

 rials for the last-named process. The Absorbents are most abundant, as 

 Lymphatics, in the Skin ; and, as Lacteals, in the Mucous Membrane of the 

 first part of the Intestinal canal ; but the Lymphatics are also largely distri- 

 buted through some of the Glandular organs. The Skin is the only part of 

 this system, which is largely supplied with Nerves ; except the Conjuctival 

 Membrane, and the Mucous Membrane of the Nose : hence the sensibility of 

 this structure is usually low, although its importance 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 subjacent layers, as 



* Cyclopaedia of Analomy and Physiology, vol. iii. p. 485. 



