300 



MUCOUS MEMBRANE. 



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 laminse 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 region 

 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 LYMPHATIC 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 

 Treviranus, 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 

 vl 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. 



Of the separation of material from the body. 

 This function appears to be carried on in 

 every part of the mucous system. One great 

 division, that of the glands, is specially des- 

 tined to it, as are likewise those portions of the 

 compound mucous membranes, which have 

 been already described as coming properly 

 under the designation of glands. If, however, 

 the essential nature of the function of secretion 

 be adequately considered, it will scarcely be 

 doubted that even the simplest parts of the 

 mucous membranes, and the whole cutaneous 

 surface (as distinguished from its sebaceous 

 and perspiratory glandular offsets) share largely 

 in this important office. It is true that in the 

 skin this function holds a subordinate place to 

 that of defence and protection, but its existence 

 is only an example of what an attentive survey 

 of nature everywhere discovers; the accom- 

 plishment of various ends by means of the 

 same simple instruments. 



The notion that a secreted product must be 

 fluid, is one that has arisen out of a partial 

 and imperfect insight into the nature of the 

 secreting process. Those matters which are 

 eliminated in the largest quantities and by the 

 largest glands are for the most part so, in the 

 shape under which they meet the eye, that is, 

 .after their separation from the organ in which 

 thev are secerned. But in the case of the lungs 

 the secretion is gaseous as well as fluid, and in 

 numerous instances, which have been recently 

 brought to light, chiefly by the labours of 

 Henle, it is found, when minutely scruti- 

 nized, to consist of organic forms entitled to 

 be styled solid. 



The problem which physiologists have now 

 to resolve, is how far these organic forms, 

 which are more or less altered epithelial par- 

 ticles, are necessarily concerned in the per- 

 formance of the function, for epithelium is all 

 but universal in the mucous system. It would 

 be foreign to the province of this article to 

 enter at length on the general question of secre- 

 tion, and 1 shall confine myself to a few re- 

 marks tending to show in what direction recent 

 researches point.* 



When the secretion of a sebaceous follicle of 

 the skin is minutely examined, it is found to 

 consist entirely of epithelial particles contain- 

 ing the sebaceous matter, and more or less 

 broken and compressed. These are similar to 

 the particles lining the follicle, and are mani- 



* Purkinje, Isis 1838, No. 7. Schwann, Froriep's 

 notiz. Feb. 1838. Henle, MUller's Archiv. 1838, 

 p. 104-8, 1839, p. 45; also Miiller's Phys. by 

 Baly, 2nd edit., vol. i., p. 503-4. 



