Epithelium of the pelvis and ureter.— The epithelium of the pelvis and ureter requires only a brief mention ; it belongs to the lamel liform or scaly variety, and consists of flat tened, delicate, transparent scales, having an tngular outline caused by their lateral appo sition, and a nucleus which is generally ec centric.
Function of the Malpighian bodies and urini fermis tubes. —Before concluding this part of our subject, it appears desirable to make some allusion to the probable office of the several parts of the kidney, whose structure has passed under review. Mr. Bowman, in the paper to which reference has so often been made, has propounded a theory as to the office of the Malpighian bodies which I believe will soon be admitted as a true and well-established doctrine, based as it is upon accurate observation, and confirmed by sound reasonino. and analogy. The theory in ques tion, and° the facts and arguments in support of it, are thus clearly stated by 111r. Bow man : — " Reflecting on this remarkable structure of the Malpighian bodies, and on their singular connection with the tubes, I was led to specu late on their use. It occurred to me that, as the tubes and their plexus of capillaries were probably, for reasons presently to be stated, the parts concerned in the secretion of that portion of the urine to which its characteristic properties are due (the urea, lithie acid, &co, the Malpighian bodies might be an apparatus destined to separate from the blood the watery portion. This view, on further consideration, appears so consonant with facts, and with analogy, that I shall in a few words state the reasons that have induced me to adopt it. I am not unaware how obscure are the regions of hypothesis in physiology, and shall be most ready to renounce my opinion, if it be shown to be inconsistent with truth.
" In extent of surface, internal structure, and the nature of its vascular network, the membrane of the uriniferous tubes corresponds with that forming the secreting surface of other glands. Hence it seems certain that this membrane is the part specially concerned in eliminating from the blood the peculiar prin ciples found in the urine. To establish this analogy, and the conclusion deduced from it, a few words will suffice. 1. The extent of surface obtained by the involutions of this membrane will by most be regarded as itself sufficient proof. But, 2. Its internal surface
is conclusive. Since epithelium has been found by Purkinje and Henle in such enor mous quantities on the secreting surface of all true glands, its use cannot be considered doubtful. It never forms less than faths of the thickness of the secreting membrane, and in the liver it even seems to compose it en tirely', for there I have searched in vain for a basement tissue, like that which supports the epithelium in other glands. The epithelium, thus chiefly forming the substance of secreting membrane, differs in its general characters from other forms of this structure. Its nu cleated particles are niore bulky, and appear from their refractive properties to contain more substance, their internal tissue being very finely mottled, when seen by transmitted light. In these particulars the epithelium of the kidney-tubes is eminently allied to the best-marked examples of glandular epithelium. 3. The capillary network surrounding the uriniferous tubes is the counterpart of' that investing the tubes of the testis, allowance being made for the difference in the capacity of these canals in the two glands. It corre sponds with that of all true glands in lying on the deep surface of the secreting membrane, and in its numerous vessels everywhere ana stomosing freely with one another.
" These several points of identity may seem too obvious to be dwelt upon, but I have detailed them in order to show that in all these respects the .Malpighian bodies differ from the secreting parts of true glands. 1. The Malpighian bodies comprise but a small part of the inner surface of the kidney, there being but one to each tortuous tube. 2. The epi thelium immediately changes its characters, as the tube expands to embrace the tuft of vessels. From being opaque and minutely mottled, it becomes transparent, and assumes a definite outline ; from being bald, it becomes covered with cilia (at least in reptiles, and probably in all classes); and in many cases it appears to cease entirely a short way within the neck of the Malpighian capsule. 3. The blood-vessels, instead of being on the deep surface of the membrane, pass through it and form a tuft on its free surface. Instead of the free anastomoses elsewhere observed, neighbouring tufts never communicate, and even the branchlets of the same tuft remain quite isolated from one another.