The mammary glands derive their extreme denseness and toughness, as well as their white colour, principally from the areolar tissue, in which the proper glandular membrane is en closed. This tissue penetrates more abundantly between the minuter subdivisions of the gland than is observed in any other instance. It thus affords support, at the same time that it permits and facilitates movement of one part of the organ on another. It is also of such a nature as to readily allow of distension during lactation.
General outline of the functions of the mu cous system.—By its external anatomical posi tion, this system is subservient to four great functions : the reception of impressions from without, the defence of the body from external injurious influences, the absorption of foreign particles, and the separation of such as are for any reason to be eliminated. It may almost be said to be the peculiar seat of these func tions, which, however, are distributed in a very unequal manner over its different regions.
Reception of external impressions.—The skin and mucous membranes appear everywhere fitted by their nervous supply to receive im pressions, which, being conveyed to the ner vous centre, may there excite a reflexion of stimulus along motor nerves, without the in tervention of consciousness. Common sensa tion, or that which in its most exalted form becomes touch, exists in all parts of the cuta neous surface, within the mouth, for some dis tance within the nostrils, and (with the excep tion of the pharynx and cesophagus) in general, wherever the epithelium is of the true scaly variety. Where the sense of touch is most perfect, the simple membrane is observed to be involuted into the form of papillae for the pur • pose of crowding a larger number of nervous loops into a given space. Taste and smell, which are nearly allied to touch, are the other special senses of which the mucous system is the seat. The sensations of hunger and thirst seem also referrible to this tissue.
Defence, rom external influences.—One chief division of the mucous system, viz. the skin, derives its main characteristics from its adap tation to this function, and those parts of the mucous membranes which are most exposed to the contact of irritating substances approach the most nearly to the skin in their structure. Their epithelium is scaly and in thick lamina., their submucous areolar tissue abundant, dense, and resisting. The nervous endowments of such surfaces, whether excito-motory or sen sorial, mainly contribute to the protection of the animal. And, on the external tegument, the developement of hairs, nails, &c. in their endless modifications of form, position, and structure, serve, with few exceptions, the same important purpose. In some parts of the mu cous membrane peculiarly obnoxious to pres sure, there are special glands for the lubrication of their free surface.
Absorption of external material.— Every particle, entering the body from without, is ab sorbed, in the first instance, through some por tion or other of the mucous system. \Vhat is now known of the nature of this function in general, renders it certain that every part of the mucous system would form an absorbing sur face, if favourably circumstanced for doing so. But as the extraneous material, to be absorbed, must be brought into contact with the absorb ing surface, often by some special and com plicated means, this function is chiefly limited to certain distinct districts of the system. With few exceptions the glands are not suited either by their position or structure to receive the contact of extraneous substances, and even many portions of the mucous membranes are incapacitated in the same manner, as, for ex ample, most of those lining the excretory pas sages of the glands. The secretions which, in a healthy state, are the only substances brought into contact with these surfaces, are, it is true, occasionally modified by a partial absorption of their constituents; but, generally speaking, this occurs to a very slight extent. Once formed, they usually traverse the channels, leading to the outlets of the body, unchanged.
The simplest condition under which this function presents itself appears to be that ex hibited by the respiratory surface, which, whe ther it be arranged as lungs or gills, is con cerned with aeriforrn particles, and absorbs and secretes through the self-same structure. The skin also is a very active absorbing surface, and appears, by the best observations, to be provided with a close net-work of lymphatics, which I have already stated to be most probably situated immediately under the basement mem brane. It does not appear that the existence of the lymphatic pores, described by MM.
Breschet and Roussel de VauAme as opening on the free surface of the cuticle, has been confirmed by any subsequent anatomist. I have sought in vain for any such system of vessels in the cuticle, and I believe those distinguished ob servers must have been deceived by the irre gular lines of union between the epidermic particles. It is true that the thickness and tex ture of the layers of epidermic scales are little calculated to allow of their being permeated by foreign material, whether fluid or gaseous ; and, therefore, it is not likely that absorption is effected to any great extent either through their substance or interstices. It seems more con sonant with facts to suppose, that this process, especially in respect to solid matters, is carried on by the simple membrane of the sudoriferous ducts, with which external particles would easily be brought into contact through their open extremities. But as these ducts traverse the thickness of the cuticle, and in that part of their course have not (in man) any proper wall, but are bounded only by the edges of the scales between which they pass, it is very probable that the deeper and softer laminae of epidermic particles may not merely be moistened by the secretion of the ducts, but, under favourable circumstances, may borrow extraneous matters from them, and thus become a part of the ab sorbing medium. In reference to the question of absorption by the skin, it is interesting to notice the modification of this structure in those lower animals in which this function is mani fested in much greater activity than in man. A better example cannot, perhaps, be selected for this purpose than that of the frog. Its epi dermis consists of a single layer of scales, and in consequence they do not overlap, but join edge to edge. These scales are not reduced to mere membrane, but always contain a con siderable quantity of fluid in their interior. The sweat-pores open here and there in the interstices between three scales, and have true walls, formed out of a pair of modified epi dermic particles, adapted to one another, and elongated into the subcutaneous texture. They thus bear a very close resemblance to the stomata of leaves. I lately discovered this singular arrangement in the cast-off cuticle of the animal. It seems undeniable, that, here, absorption is effected by the whole series of epidermic scales, as well as by the pores. . But the most remarkable, and at the same time the most recondite form, under which this func tion is exhibited in the mucous system, is that met with in the alimentary tract. Here, indeed, water and aqueous solutions are imbibed, with great rapidity, into the vascular plexuses of the blood and lacteal systems, as the united testi mony of many able experimenters abundantly shews. But from this merely physical process of imbibition is to be distinguished the more mysterious and elective function of chylous absorption, which is conducted by the lacteals alone, and is consequently limited to the recion supplied with that system of vessels. For an account of the present state of knowledge on the highly important subject of the intimate nature of this function, the reader is referred to ABSORPTION and LYMPIIATIC SYSTEM, in which he will find the chief of the conflicting statements and opinions of physiologists de tailed and discussed. It has already been ex plained in the present article, that the latest observations on the structure of the villi, and apparently the most exact ones, because con ducted with the most improved lenses, and accordant with other collateral discoveries, make it highly probable that the opinion assigning open mouths to the lacteals is erroneous. In the description of those orifices, furnished by Treviranns, we may plainly discern his partial acquaintance with characters which we now know to be those of the prismatic epithelium investing the villi ; and the less precise asser tions of the same kind by several other excel lent anatomists, we may now, perhaps, fairly consider to have been founded on deceptive appearances which, in their day, did not admit of accurate interpretation. If any such orifices exist, their minuteness must be extreme, and they must lie in the intervals between the prisms of epithelium. But even such attenu ated pores, the best microscopes fail to detect, and at least it may with certainty be affirmed, that none large enough to admit a chyle-globule exist. The structure of the villi, no less than our knowledge of the absorbent function in general, seems to indicate that the chyle, when first taken up, is strictly a fluid, and only ac quires its solid particles after it has entered the lacteal plexus.