MUCOUS MEMBRANE. 



489 



but which is quite distinct from the cuticle, 

 and the great muss of that complicated struc- 

 ture to which the terms ' cutis ' and ' derm is ' 

 are applied. 



A very strong reason for believing this mem- 

 brane to be present in the skin, is the fact of 

 its existence in those minute organs, so profusely 

 scattered under the cutaneous surface, the se- 

 baceous and sudoriferous glands. In Jig. 277 

 I have represented it in a portion of one of the 

 latter, taken from the axilla, where they are 

 very large. These glands are nothing more 

 than involutions of the external tegument, and 

 correspond closely with the labial and allied 

 glands connected with the ordinary mucous 

 membranes. It is impossible to suppose that 

 a structure attaining so marked a developement 

 in those parts, should be wanting in the general 

 superficies, with which they are, at numberless 

 points, directly continuous. 



In other situations, where a simple expanse 

 of mucous membrane is spread out upon a 

 surface of the body, as in the oesophagus, 

 pharynx, mouth, nose and its sinuses, vagina, 

 bladder, &c. (from all of which, however, 

 there are numerous prolongations called follicles 

 and glands, which shew this structure well,) 

 a basement tissue such as that described has 

 not been shown to exist. Its existence rests 

 at present principally on analogy, and it is 

 difficult to say whether it be not more or 

 less modified. Certain of the peculiarities 

 presented by these several parts depend on 

 a modified form and greatly augmented mass 

 of the epithelial element, but many also on 

 varieties in the areolar and vascular tissues 

 underlying the mucous tissue, and, properly 

 speaking, forming no part of it. These will 

 be treated of under the topographical descrip- 

 tion of the membrane. 



Of the epithelium. A very brief period has 

 elapsed since it was universally held that most 

 mucous membranes wanted epithelium, and 

 their analogy with the skin was only maintained 

 in this particular by a fancied resemblance 

 drawn between epidermis and mucus. One of 

 the principal results of microscopic observation, 

 conducted with the improved modern instru- 

 ments, is that of Ilenle, proving not only that 

 this structure is present throughout the mucous 

 system, but that in most situations it is so abun- 

 dant as to constitute nearly the whole material of 

 the tissue. This fact, as yet so novel, coupled 

 with the discovery announced at the same time 

 of the occurrence of a lining of analogous cha- 

 racter on all internal cavities, makes the study 

 of this structure under its varied forms pecu- 

 liarly interesting and important. It will readily 

 be conceived how wide a field is here opened 

 to view, and how premature it would yet be to 

 attempt to offer a general history of such a 

 structure. The numerous questions presenting 

 themselves on every side render this impossible; 

 and if it were not so, the scope of the present 

 article would oblige me to confine the descrip- 

 tion to those forms of epithelium met with 

 in the mucous system. In acknowledging the 

 great obligations I am under to Henle's admi- 

 rable paper on this subject, I may state that 



the following account has been written as much 

 as possible from my own observations. 



By the term epithelium is now meant a layer 

 of particles or modified cells, furnished with 

 nuclei and nucleoli, lining an internal surface 

 of an organized body, and by their apposition 

 and union constituting a kind of pavement. 

 A similar investment to an external surface is 

 styled epidermis. Both these, in their ordinary 

 forms, will be embraced by the following de- 

 scription. 



Epithelium is an organized structure endowed 

 with vitality. This is shewn by its form, the 

 process of its growth, and the living properties 

 it displays. Of these the most eminent is that 

 of ciliary motion, which in all the higher 

 animals is performed by cilia clothing the free 

 surface of epithelial particles. But in very 

 many situations, if not in all, the processes 

 of nutrition carried on in the epithelial layer 

 of the mucous system differ materially from 

 those of other organic tissues ; the old elements, 

 which in other cases are reconveyed into the 

 blood, being here shed on the free surface of 

 the membrane, and thus becoming at once 

 eliminated from the system. 



The epithelial particles preserve a greater 

 resemblance to the form of the development 

 cell than most other tissues. In many parts 

 they continue to be truly cells throughout their 

 existence, and in no instance is the nucleus, 

 from which they have proceeded, absorbed. 



In connection with a wide and varied range 

 of function, these particles present numerous 

 modifications of form, bulk, and texture, the 

 leading features of which have been pourtrayed 

 by Henle. The following arrangement, how- 

 ever, differs in several respects from that pro- 

 posed by him,* and is more in accordance with 

 what I have myself observed. Founding it on 

 the anatomical condition of the particles and 

 on their office, I distinguish three varieties, 

 the latnellij'orm or scaly, the prismatic, and the 

 spheroidal. These all run together by imper- 

 ceptible gradations. The particles may be also 

 divided into non-ciliated and ciliated, the scaly 

 being always bald, the prismatic and sphe- 

 roidal in some situations furnished with cilia. 



Of the lumetliform or scaly variety. This 

 consists of broad flattened particles (or scales, 

 properly so called), having an angular outline 

 (caused by their lateral apposition) and a 

 nucleus, which is generally eccentric. These 

 scales form layers of extremely variable thick- 

 ness. They are generally, however, super- 

 imposed in great numbers over one another, 

 as in the mouth, fauces, and oesophagus of the 

 human subject, where they constitute the 

 opaque defensive investment so visible to the 

 eye in those parts. 



But the best-known example of this form is 

 presented by the cuticle, which from its ex- 

 posed position is thicker and denser than any 

 internal epithelium. This variety, then, is the 

 one which offers the most convincing proof of 



* He divides it into pavement epithelium (or 

 the scaly), cylinder epithelium (or the prismatic), 

 and ciliated epithelium. See Miiller's Archiv. 

 1838. 



