But in numerous instances besides those that have been mentioned, there is more or less direct evidence of an actual shedding and con tinual renovation of the epithelium. The scaly variety of this tissue, whether on skin or mu cous membranes, is a wide-spread example of this: the particles may be observed to augment in size by the intus-susception of new material from the blood, afterwards to undergo a slow loss of substance, and, finally, to lose their connection with the body altogether. They retain their position till nothing but the nucleus and cell-membrane remain, till they are re duced, as it were, to a mere skeleton. How the material thus separated from the body is to be distinguished from a secretion, it would not be easy to decide. In the saliva of the mouth, are present, not only detached scales, but globu lar nucleated particles, of a very delicate aspect and regular character, which seem manifestly to come from the salivary glands. They differ in some respects from the epithelium of these organs, but appear most probably to be par ticles of it altered by endosmose of the water of the secretion through the cell-membrane ; for the ultimate vesicles and ducts of these glands are not merely lined, but filled, with epithelial particles, which, being thrown off front the basement membrane, must in due time escape to make room for the advancing series : and yet none of them in an unaltered state are found in the saliva.
I may in this place refer to an opinion recently entertained in Germany, that the secreting membrane of certain glands is arranged in the form of closed vesicles filled with nu cleated particles, which, from time to time, are discharged, as the secretion, by the bursting of the cell in which they are contained. Henley conceives that this arrangement exists in the mammary, salivary, and lachrymal glands, as well as in almost every mucous membrane, however apparently plain and simple. Was mannt has described a similar structure in the middle part of the stomach of the pig. This view of the existence of closed vesicles is obviously at variance with the general view before given of the universal continuity of the simple membrane of the mucous system. I am familiar with many of the appearances on which it is founded, and without presuming to pronounce them very decidedly deceptive, I may state that hitherto my observations induce me to agree with Dr. Baly in his rejection of the interpretation put upon them by the Ger man anatomists. A thin slice of a mass of the many-lobed terminal vesicles of one of these glands, especially if compressed, very readily assumes the aspect of a congeries of cells, each entirely surrounded by an envelope of base ment membrane. But I have several times, in favourable sections, observed this membrane passing off into a neck, and becoming con tinuous with that of the duct. Such observa tions seem to me in a great measure conclusive on this subject ; and I am strengthened in this view by the fact, that the capsules of the Malpighian bodies of the kidney are now universally considered to be perfectly closed vesicles, whereas they are in reality the ex panded wall of the duct, as I have lately shown by several kinds of proof.* But what ever may be the real fact in the matter under dispute, it is admitted by all that the epithelium is formed in enormous quantities, and is being continually thrown off; which is the circum stance chiefly intended to be insisted on at present.
In the healthy bile also, in the urine, and in various other secretions Dr. lienle has met with particles of epithelium detached from the excretory passages, and in different stages of decay.
Turning to those two great emunctories, the liver and kidneys, in the secretions of which no trace of the epithelium of the secreting part of the organs can be detected, we might be disposed, on a slight consideration, to conclude the evidence they furnish to be unfavourable to the general position here advanced. We must,
indeed, be content for the present to acknow ledge that it is less plain and direct, and shrouded in our great ignorance concerning the play of chemical affinities in living bodies; but still it is too interesting and important to be passed over in silence. Though the epithe hum of these organs be not detached entire, as in many other cases, there is much, in each instance, to explain the discrepancy consistently with the theory in question.
I have described the lobules of the liver as consisting of a solid plexus of capillary blood vessels, in the meshes of which is a congeries of epithelial particles. We possess no accurate account of the mode of termination of the biliary ducts; but it seems clear, from the small meshes of the vascular plexus being completely filled by the epithelium, that no true ducts, i. e. tubes, penetrate the substance of the lobules : the tubular ducts probably commence on the surface of the lobules. The epithelium of the lobules is doubtless conti nuous with that of the ducts, but the cavity of the ducts and their basement membrane termi nate at the surface of each lobule. Though the cavity of the ducts be not continued within the lobule, yet it is very possible that injection urged along the ducts might insinuate itself by the side of the epithelium into the interstices of the vascular plexus, and thus, like the epithelium itself, form a solid plexus within the lobule. This appearance probably led Mr. Kiernan to describe the termination of the ducts as forming a plexus within the lobule, the lobular biliary plexus. And this description must be allowed to be essentially correct ; for although the cavity of the duct cease at the surface, the epithelium of the lobule is, in respect of function, its real continuation. I have further observed, that although the epithe lium of the lobule has, on the whole, a plexi form arrangement, yet its particles in some measure affect a radiating direction from the central axis towards the circumference, perhaps towards certain parts only ; and when a lobule is broken up by violence, the resulting frag ments of epithelium are apt to consist of a linear series of particles. Many of the particles, too, are smaller than the rest, and have all the appearance of having been recently formed and as yet incompletely developed. It is also remarkable that the particles should contain granules of oily matter in their interior; for although chemical analysis has detected diffe rences between this substance and cholesterine,* yet as the chief peculiar principles of the bile are forms of hydro-carbon, the coincidence cannot be an accidental one. It is not con tended that the contents of these particles are the finished secretion, but rather that their chemical constitution undergoes some modifi cations during the disintegrating process. And it is worthy of notice, that in many cases where the decarbonizing function of the lungs is slowly but greatly interfered with, as in phthisis pulmonalis, and where the liver is consequently called into increased activity as a compensating organ, these oily globules exist in such abun dance and size as to gorge and swell the parti cles (and therefore the whole viscus) to nearly double their natural bulk.t But this is not all the evidence, that this epithelium is the source of the bile. I am informed by my friend, Dr. W. Budd, that Dr. Henle in his recent edition of Soemmering, of which I have not yet been able to obtain a copy, describes the epithelial particles as appearing yellow or yellowish brown in direct light, and as probably containing bile. Ile also states that the presence of the fatty globules in the epithelium is inconstant, and corresponds with the varying fatty contents of the bile. Ile is unable at present to determine in what manner the contents of the particles find their way into the ducts.