234 OF THE PRIMARY TISSUES OF THE HUMAN BODY. 



opaque mucus, discharged from membranes in a state of irritation or inflamma- 

 tion, these corpuscles are present in greatly increased amount ; and cells are 

 often developed around them. 



229. The essential character of the Mucous Membranes, in regard alike to 

 their offices and their arrangement, is altogether different from that of the Serous 

 and Synovial membranes. For whilst the latter form shut sacs, whose contents are 

 destined to undergo little change, the former line tubes and cavities which have 

 free outward communications; and they thus constitute the medium through 

 which nearly all the material changes are effected that take place between the 

 living organism and the external world. Thus, in the gastro-intestinal mucous 

 membrane, we find a provision for reducing the food, by means of a solvent fluid 

 poured out from its follicles; whilst the villi, or root-like filaments, which are 

 closely set upon its surface towards its upper part, are especially adapted to absorb 

 the nutrient materials thus reduced to the liquid state. This same membrane, 

 at its lower part, constitutes an outlet through which are cast out, not merely 

 the indigestible residuum of the food, but also the excretions from numerous 

 minute glandulae in the intestinal wall, which result from the decomposition of 

 the tissues, and which must be separated and cast forth from them to prevent 

 further decay. Again, the bronchio-pulmonary mucous membrane serves for the 

 introduction of oxygen from the air, and for the exhalation of water and car- 

 bonic acid. And lastly, the mucous membranes are continuous with the cell- 

 lined vesicles or tubes of the various Grlands, which are the instruments whereby 

 their respective products are eliminated from the blood. The changes to which 

 the Mucous Membranes are thus subservient, however, do not seem to involve 

 the vital activity of any other of their components than the Epithelial cells, 

 and of the basement membrane as probably ministering to their production. 

 Here, as elsewhere, the fibrous elements appear to have but a passive relation 

 to the vital operations of the tissue into which they enter; and there is no rea- 

 son to think that the copious supply of blood which the mucous membranes 

 receive has any relation to their nutrition. In fact, we might fairly describe 

 the Mucous membranes generally as essentially consisting of a plexus of blood- 

 vessels in immediate relation with a stratum of epithelial cells; the fibres having 

 merely a connective office, and their absence not being in any way detrimental, 

 if they be not required for this purpose. Thus, the tubuli and follicles of many 

 glands are composed of a basement membrane and epithelial layer, prolonged 

 from those of the mucous membranes with which they are in connection, and 

 yet may have no fibrous tissue properly appertaining to them, being imbedded 

 in the substance of the glands, and closely surrounded by bloodvessels. Mucous 

 membranes are not, for the most part, copiously supplied with nerves, nor do 

 they possess much sensibility; there are exceptions, however, chiefly in the case 

 of those which, being near the inlets and outlets of the body, are endowed with 

 sensibility, apparently for the purpose of guarding against the contact or admis- 

 sion of injurious substances (as in the case of the conjunctival, buccal, and 

 laryngeal membranes), or of giving notice of the presence of excrementitious 

 matters requiring ejection by muscular power (as in the case of the lining mem- 

 branes of the bladder and rectum). Mucous membranes, when diseased, are far 

 less disposed than the serous to throw out plastic exudations, but are prone to 

 suppuration, ulceration, and gangrene. Their regeneration after loss of sub- 

 stance by disease or injury, takes place with great rapidity ; but although a 

 simple membrane may be 'completely restored, yet it appears from observation 

 of the healing process after ulcers of the large intestine, that the tubular folli- 

 cles are not reproduced. A complete reproduction of the follicular structure 

 takes place, however, in the lining membrane of the uterus, after its exuviation 

 in the formation of the Decidua (CHAP. xix.). It is interesting to observe, 

 that where a portion of the Cutaneous surface has been turned inwards, so as to 



