MUCOUS MEMBRANE. 



485 



ally gaining ground, that all the glands fur- 

 nished with excretory ducts have a very close 

 relation to the former, in which their ducts for 

 the most part open. Still, it does not appear 

 that the proofs of this alliance have heen hi- 

 therto, by any author, deemed sufficient to in- 

 duce him to blend these several parts under a 

 description common to them all. Even M tiller, 

 in whose philosophical work on the glands is 

 contained so much new and important evidence 

 of this relation, continues thus to sever them 

 in the late edition of his Physiology- But, in- 

 deed, although much weight is to be granted 

 to the arguments drawn from continuity and 

 occasional convertibility of structure, course of 

 developement, rough analogies of composition 

 or of function, and sympathies under disease, it 

 must be allowed that hitherto that most im- 

 portant of all proofs has been all but wanting, 

 which, as I shall endeavour to show, is capable 

 of being derived from minute anatomical ana- 

 lysis. 



The researches which I have hitherto been 

 able to make on this subject are still so incom- 

 plete, that I should have gladly delayed their 

 publication for some time longer, had the pro- 

 gress of this work admitted of it. As it is, I 

 shall state the conclusions to which I have 

 been led, and the grounds they rest upon, 

 (pointing out, as far as possible, where farther 

 examination is demanded,) with the hope of 

 thereby giving a clearer and more satisfactory 

 view of the structure and relations of this im- 

 portant class of tissues than could be otherwise 

 accomplished. 



I shall point out that the skin, mucous mem- 

 branes, and secreting glands, consist of certain 

 elements, which the anatomist may detect and 

 discriminate, some of which are essential to 

 their tissue, others appended or superadded, 

 and that the broad characteristic distinctions 

 between these structures, appreciable to ordi- 

 nary sense, as well as the innumerable grada- 

 tions by which they every where blend insensi- 

 bly with one another, are solely due to various 

 degrees and kinds of modifications wrought in 

 the form, quantity, and properties of these re- 

 spective elementary parts. 



The skin is the outer tegument of the body ; 

 the mucous membranes form its internal invest- 

 ment, and are continuous with the skin. The 

 ducts of all glands are continuous either with 

 skin or mucous membrane, and their true 

 secreting portion, as already described, (see 

 GLAND,) is merely a further prolongation of 

 the same tissue. These offsets, like the great 

 mucous tracts, are in the direction of the inte- 

 rior of the body ; they form follicles and tubes 

 of infinite variety, and. however complicated, 

 may still be regarded, in a certain sense, as ex- 

 ternal to all other textures. Thus the mucous 

 system may be described as a great and un- 

 interrupted membrane, every where perfectly 

 closed, in which the rest of the animal, or the 

 parenchyma, is enclosed. This membrane has 

 two surfaces, the one free, superficial or exter- 

 nal, the other attached, deep, or parenchymal. 

 It is on the parenchymal surface that the ap- 

 pended structures (viz. blood- and lymphatic 



vessels, nerves, and areolar tissue) are found 

 in more or less profusion. 



The functions of the mucous system, nume- 

 rous and diversified as they are, all bear a dis- 

 tinct reference to its really external anatomical 

 position, and by this circumstance they are 

 associated together: the principal are sensation, 

 absorption, secretion, excretion, and defence of 

 the parts lined by it against the contact of 

 foreign bodies. 



A glance at the opinions that have prevailed 

 concerning the structure and relations of the 

 mucous membranes, will exemplify, more clearly 

 perhaps than any other course, how imperfect 

 have been the means employed, until a very 

 recent period, in researches into minute or 

 structural anatomy. The distribution of their 

 bloodvessels had indeed been studied with 

 brilliant success by liuysch, Lieberkuhn, and 

 others, by the aid of injections, the admirable 

 delicacy of which no modern art has surpassed ; 

 and somewhat of their extensive connexions, 

 general properties, and even of their texture, 

 had been divined from rough dissection, mace- 

 ration, and observations on the mode of their 

 developement and on their morbid states. But 

 the ignorance that really prevailed, as to their 

 intimate structure, is abundantly evinced by the 

 number of disputed questions, the absence of 

 precision of detail, and the substitution of loose 

 and unwarranted analogies in its stead. Within 

 the last five years discoveries have been made 

 which throw a new and most important light 

 on the whole subject, and when viewed in con- 

 nection with one another, must be considered 

 to have greatly simplified our knowledge re- 

 specting it. These discoveries, due chiefly to 

 Boehm, Boyd, and Henle, result from exami- 

 nations of recent specimens with the micro- 

 scope, and those of the last observer, which are 

 especially valuable, were made with high pow- 

 ers employed upon a single tissue (the epithe- 

 lium) in different forms and situations. It is 

 this kind of research that promises the most 

 enlarged and trustworthy results to any one 

 who will follow it in a spirit of due caution 

 against hasty generalization, and which has 

 already done so much in the present day to- 

 wards a complete remodelling of our ideas, 

 both concerning the elements of organization 

 and their union to form compound tissues. 



Before proceeding to a description of the 

 anatomical elements of the mucous system, it 

 is necessary to premise that a great portion of 

 the membranes, usually termed mucous, are 

 glands of a complicated structure, arranged in 

 a membranous form, consisting of a closely 

 packed mass of secreting tubules, which open 

 on the general surface, and are essentially invo- 

 lutions of it. The bloodvessels and other ap- 

 pended tissues occupy the intervals of these 

 tubules, and so approach the surface ; but, ne- 

 vertheless, they always remain on the deep or 

 parenchymal aspect of the mucous tissue. So, 

 the same membranes present projections, which 

 are nothing more than hollow evolutions of the 

 same mucous tissue, into which the appended 

 tissues are extended. The same remarks apply 

 strictly to many regions of the skin. Hence it 



