MUCOUS MEMBRANE. 



495 



Where mucous membranes are not destined 

 to move on the parts they cover, the areolar 

 tissue beneath them is very scanty. This is 

 the case in the nasal cavities, even in the por- 

 tions furnished with a great substratum of 

 bloodvessels. But where much motion is re- 

 quired, as where a muscular lamina underlies 

 the mucous, and the enclosed cavity is liable to 

 vary in its dimensions, the areolar tissue is co- 

 pious, and very similar in its elements and in 

 the size of its interstices to the ordinary forms. 

 Examples of this are seen in the whole alimen- 

 tary tract. 



But it is under the cutaneous part of the mu- 

 cous system that this tissue assumes its highest 

 developement. Elsewhere its object is to pro- 

 mote freedom of movement, or to confer elasti- 

 city. Here it answers both these purposes, and 

 in addition gives a great capacity of resistance 

 against external pressure and violence. The 

 former end is attained by the structure called 

 subcutaneous fascia, which is a large quantity 

 of this tissue in its ordinary form. The two 

 latter are effected by that more condensed part 

 to which the term of cutis has been given. This 

 last is the structure to which the submucous 

 areolar tissue of the intestinal canal mainly cor- 

 responds, as may be shown by an examination 

 of the submucous tissueof the mouth, pharynx, 

 and oesophagus, which holds an intermediate 

 place. To describe its modifications in different 

 situations would be to encroach too much on 

 the province of another article (see SKIN), and 

 a few general remarks must here suffice. 



The framework of the cutis may be said to 

 consist entirely of a modified form of the areolar 

 tissue. Both elements are enormously deve- 

 loped, but especially the yellow fibrous one. 

 The fibrillae of this are thicker than elsewhere, 

 and branch and inosculate with great freedom, 

 enclosing interstices open on all sides, and 

 giving passage to the wavy bands of the white 

 fibrous element as well as to vessels, nerves, 

 the ducts of the sweat-glands, the sebaceous 

 glands, and the roots of the hairs. These in- 

 terstices are in general very close, but they vary 

 with the size of the parts which occupy them. 

 On the deep surface of the cutis the yellow 

 fibrous element changes gradually into that of 

 the subcutaneous fascia, or that of ordinary 

 areolar tissue. It cannot be doubted that the 

 skin chiefly owes its elasticity and toughness to 

 this remarkable developement of the yellow 

 fibrous element. 



Topographical view of the mucous system in 

 wan. Keferring the reader to the article SKIN 

 for a detailed description of that part of the 

 mucous system, and its immediate dependen- 

 cies, I shall now proceed to point out some of 

 the more remarkable varieties of the internal 

 tracts. These tracts have been usually com- 

 prehended under two general divisions, the 

 gastro-pulmonary, and the gemto-urinary. The 

 former is continuous with the skin at six points, 

 the two eyelids, the two nostrils, the mouth, 

 and the anus; the latter at a single one, the 

 orifice of the urethra in the male, and the labia 

 pudendi in the female. Besides these, there 

 are two smaller tracts, the mammary, each of 



which is subdivided into several, which open 

 separately on the skin. 



The description of the gastro-pulmonary 

 tract may be commenced at the lips. It covers 

 their inner surface, the cheeks, gums, tongue, 

 and palate, and extends into the labial, buccal, 

 and larger salivary glands, of which it consti- 

 tutes the chief mass. It passes over the arches 

 of the palate, (where its involutions form the 

 tonsils, )and lines the pharynx, Eustachian tubes, 

 and the cavities of the tympana. Penetrating 

 into the nose by the posterior nares, it lines 

 all the passages and chambers of that organ, 

 and advances along the nasal duetto the lachry- 

 mal sac. Thence it may be traced along the 

 canaliculi to the front of the eye, where it takes 

 the name of tunica conjunctiva; covers the 

 posterior surface of the eyelids, a certain por- 

 tion of the sclerotic, and the cornea, and forms 

 the caruncula, the Meiboraian and lachrymal 

 glands. In these complicated portions of its 

 course, the membrane shares more or less in the 

 construction of the five organs of special sense, 

 and is the essential seat of two of them, taste 

 and smell. From the pharynx it spreads in 

 two directions ; first, into the larynx, trachea, 

 tracheal glands, and bronchial ramifications, 

 until it terminates by forming the air-cells of 

 the lungs ; secondly, into the alimentary 

 canal. Here it lines the oesophagus, stomach, 

 and intestinal tube, as far as the anus, and it 

 penetrates along the excreting ducts of the liver 

 and pancreas, into the inmost recesses of those 

 glands, to form their secreting surface. 



The genito-urinan/ tract may be traced along 

 the urethra into the bladder, ureters, and pelvis 

 of the kidneys; and thence into the substance 

 of those organs as far as the Malpighian bodies, 

 the extremities of the uriniferous tubules.. In 

 connection with the urethra, processes pass to 

 the glands of Cowper; and, in the male, 

 into the interior of the prostate, the vesiculac 

 seminales, vasa deferentia, and tubules of the 

 testes. In the female, the vagina, uterus, and 

 Fallopian tubes receive a lining from it, which, 

 at the fimbriated extremity of those canals, be- 

 comes continuous with the serous membrane of 

 the abdomen.* 



The very remarkable differences presented to 

 the eye by different parts of this system have 

 been a source of great difficulty to anatomists, 

 who, on other grounds, believed them to be 

 nearly allied ; and it would appear that hitherto 

 no satisfactory explanation has been given of 

 the anatomical conditions on which this variety 

 depends. This deficiency I sh;ill now endea- 

 vour in some degree to supply. From the ex- 

 aminations I have made, I have been led to 

 consider in a distinct and separate manner the 

 several elementary tissues already mentioned, 

 composing the simple mucous membrane, and 



" This remarkable exception to a general fact 

 has long attracted attention. As a mere anatomi- 

 cal difficulty, it has lately received curious illustra- 

 tion from Hcnle's discovery of the existence of an 

 epithelium on serous and other allied surfaces. But 

 its true explanation can probably only be attained 

 by a study of its morphology, joined with that of its 

 final cause. 



