MUCOUS MEMBRANE. 



499 



suit from the predominance of their epithelial 

 element over the others. The ducts terminate in 

 vesicles, very similar in the figure they assume 

 to those of the lungs, but nearly filled up 

 with epithelial particles. The basement mem- 

 brane is very delicate. The capillary vessels 

 encircle the vesicles, and are comparatively few 

 in number, whence the pale colour of these 

 glands. The areolar tissue forms capsules for 

 those aggregations of vesicles, termed lobules, 

 but does not penetrate between the individual 

 vesicles. 



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 junctions of the mu- 

 cous si/stem. 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 nostril?, and (with the excep- 

 tion of the pharynx and oesophagus) 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 from 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 laminae, 

 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. What 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 stale, 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 aeriform 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 Vauzeme 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 seerns 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 



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