MUCOUS MEMBRANE. 



503 



apparatus at the extremity of each secreting 

 tube, apparently designed to furnish a flow of 

 water down the canal.* A large quantity of 

 water is evidently required in this secretion as 

 a menstruum for the salts and proximate prin- 

 ciples it contains; and there is no doubt, from 

 the analogy of other glands, that the walls of 

 the tubes are the membrane secreting these 

 substances. Now the epithelium constitutes at 

 least Jths of their thickness, and is the only 

 part of them with which the water can come 

 into contact. It therefore seems highly pro- 

 bable that this fluid is provided in the manner 

 described, in order to dissolve, out of the 

 epithelial particles, the peculiar principles which 

 they have previously assimilated from the 

 blood. 



In support of this general position it may 

 be observed, further, 1. That the epithelium, 

 which constitutes so large a portion of the true 

 glands, is solid and bulky, usually character- 

 ized by its finely granular texture, and in this 

 respect contrasts strongly with that lining the 

 vascular system, which is of extreme delicacy 

 and transparence. The exceptions to this re- 

 mark confirm its importance. In the air-cells 

 of the lungs, the secretions of which are ga- 

 seous and not solid, the epithelium is of great 

 tenuity, and in the Malpighian capsules of the 

 kidney, which appear to serve principally as 

 receptacles for the aqueous fluid that escapes 

 from the bare capillaries within them, this 

 structure is either wanting or consists of per- 

 fectly transparent particles. In many inter- 

 mediate varieties, too, there appears traceable a 

 correspondence between the bulk of the nu- 

 cleated particles and the activity of the secreting 

 function ; of which the scaly form in general 

 may be mentioned as an instance. 2. That 

 many peculiar substances are secreted into the 

 interior of nucleated cells, although prevented, 

 by the position of those cells, from escaping 

 from the body. Such are various fats and fixed 

 oils, colouring matters, &c. 3. That this func- 

 tion of abstracting somewhat from the blood, 

 and elaborating it, seems the most probable 

 one that can be assigned to the thymus and 

 thyroid bodies, the spleen, and supra-renal 

 capsules, and specially to the nucleated par- 

 ticles forming so large a portion of these several 

 structures. On the whole there seems much 

 weight of evidence in favour of the proposi- 

 tion " that secretion is a function very nearly 

 allied to ordinary growth and nutrition : that 

 whereas these are a combination of two func- 

 tions, assimilation of new particles and rejection 

 of old, the old being reconveyed into the blood, 

 secretion consists in a corresponding assimila- 

 tion and rejection, but the old particles are at 

 once thrown off from the system without re- 

 entering the blood. According to this view, all 

 effete material received into the blood, from the 

 old substance of the various organs, must be 

 reassimilated by an organized tissue, specially 

 designed for the purpose, viz. the epithelium, 

 before it can be eliminated : and all substances 



* Phil. Trans. Pt. I. 1842, p. 73. See also the 

 article RliN. 



thrown off from the system, but designed for 

 an ulterior purpose, must in like manner be 

 assimilated in order to their separation." It 

 places in a strong light a principle of great im- 

 portance in physiology, the subordination of 

 the bloodvessels and their contents to the tis- 

 sues among which they are distributed. 



The function of secretion may therefore be 

 considered to be universal over the mucous 

 system, and its different activity in various si- 

 tuations to be dependent on, as it certainly is 

 closely associated with, differences in the ar- 

 rangement and structure of the epithelial ele- 

 ment. The basement membrane, from being 

 absent from the lobules of the liver, seems a 

 tissue of inferior (perhaps of no) importance 

 in respect of this function, and probably is 

 chiefly subservient, wherever it exists, to the 

 mechanical support of the epithelium. 



There are probably three ways in which the 

 secretions are finally separated from the body : 

 and these three ways appear to have a reference 

 to the chemical qualities of the product, and to 

 their effete or non-effete character. 1. The par- 

 ticles assimilated into the nucleated cell may 

 be thrown off by virtue of minute chemical 

 changes occurring in it, without the cell itself 

 being altered in form. In this case the nu- 

 cleated cells will be permanent, or only very 

 slowly renewed, and the secretion will be 

 formed, or at least perfected, by the passage 

 of its elements through the cells. 2. The nu- 

 cleated cells, as they arrive at their full size, 

 may undergo a slow change in the arrangement 

 of their elements, and gradually disappear by 

 a kind of solution or deliquescence, thus form- 

 ing the secretion. 3. The nucleated cells, when 

 mature, may be cast off at once, and entire, 

 with their contents. The two last modes are 

 attended with a continual formation of new 

 cells. 



It would appear that, in general, where the 

 secretion is formed by the rejected chemical 

 elements of the cells (1), or by the destructive 

 solution of the cells (2), it is effete; but that, 

 when formed chiefly by the separation of cells 

 that are mature and contain much organic 

 matter (3), it is destined for ulterior purposes 

 in the acunomy. Of the first the kidney seems 

 to be an example, of the second the liver, of 

 the third the lining membrane of the stomach. 



The varieties in the qualities of the products 

 secerned by different portions of the mucous 

 system are only referrible to varieties in the 

 elective powers of the tracts which respectively 

 furnish them, and admit of being most readily 

 explained by the view of the nature of secre- 

 tion already advanced. It is unnecessary, in 

 this place, to enter on a particular description 

 of the boundaries of these several tracts, and 

 I shall only offer a few observations on the 

 nature and extent of that secretion which has 

 given its name to the structures here treated of. 



The term mucus, like so many others trans- 

 mitted from an early period, was originally 

 employed to denote an exaggerated and partial 

 condition, was subsequently applied more 

 loosely and widely in a generic sense, and 

 now requires to be reduced to a more definile 



