The general course and mode of division of the tubes, as well as their connexion with the Malpighian bodies, is best ascertained by the examination of specimens in which the tubes have been filled by injection ; but our knowledge of the essential structure of the tubes, and particularly of their epithelial lining, would be very incomplete without a careful examination of uninjected speciraens with a high magnifying power.
illode of injecting the Tubes.— The tubes may be more or less completely injected in two modes : 1st, by a liquid thrown into them from the pelvis of the kidney ; and 2dly, by the extravasation of materials forcibly in jected into the blood-vessels of the Malpighian bodies. By the first mode the injected ma terials are made to enter the open mouths of the tubes at one extremity, and to pass towards the other, which, as will presently be shown, is a closed extremity ; while by the second method the injection is admitted into the closed extremities of the tubes, whence it flows towards their open mouths, and so in some instances escapes into the pelvis of the kidney. By the last mode the tubes are often completely filled from one extremity to the other, while by the first method of injection they are generally very imperfectly- filled, and this even when the air-pump has been used to aid the flow of the injection into the tubes. A consideration of the structure and relation of the tubes will show that this result is a neces sary consequence of the anatomical dispo sition of the parts. Mr. Bowman remarks *, " To those who are acquainted with the practical difficulties of the injection of the ducts of glands in general, and especially of those which are very tortuous, the following considerations on this subject will probably appear conclusive. Even of the testis (where the tubes are far thicker and stronger in their coats, and much more capacious than in the kidney), there are not ten specimens that can he pronounced at all full in the museums of Europe ; and there is no evidence that, even in the best of these, the injected material has reached the very extremities of the tubes. In the kidney, the tubes are exceedingly tor tuous after leaving the Malpighian bodies, and only become straight, in most animals, in pro ceeding towards the excretory channel to discharge themselves. The way towards their orifices is so free in a natural state, that their fluid contents exert no distending force upon their walls. Accordingly their walls are ex ceedingly feeble; the basement membrane on which their strength -mainly depends is very delicate, and easily torn. they are therefore incapable of offering much resistance to a fluid impelled into them from the pelvis, but burst easily if it be forcibly urged. But were the coats ten times as tough as they really are, injection could not penetrate far into their convoluted portion, unless pushed with much force; and this for two reasons: 1st, the fluid which the tubes already contain has no means of escape before the injection, since these canals end by blind extremities in the Mal pighian bodies ; 2dly, the layer of epithelium is, immediately after death, very prone to separate from the basement membrane which it lines, and to fall into and block up its narrow channel." It consequently happens that, in face of mechanical obstacles such as those above mentioned, the force employed to in ject the tubes sooner or later bursts their coats ere their extremities have been reached.*
Course and Termination of the cing the tubes from the apex of a medullary cone, on the surface of which their open mouths may be seen, we find them taking a straight course through the pyramid, branch ing dichotomously and diverging as they pro ceed (fig. 143). After reaching the base of the pyramid, their course through the cortical portion is very various ; many tubes imme diately take a very tortuous course, some of them bending down into the inter-pyramidal portions of the cortical substance, while those near the centre of the pyramid pass onwards in a mass and in a straight line towards the surface, the tubes on the sides of the bundle in their progress passing off successively in a tor tuous course through the cortical substance, so that only a few of the central tubes in each bundle retain their straight course quite up to the surface of the kidney ; these finally turn backwards, making many convolutions in the cortical substance. After leaving the medullary cones, the branching of the tubes, except in very rare instances, appears to cease ; occasionally two tubes in the cortical sub stance unite in passing towards the cones ; and I once saw this occur at a very short distance below the Malpighian bodies, so that two short tubes, each with a Malpighian body at its extremity, united into one common tube. Some distinguished anatomists have main tained that the tubes, after dividing in the corti cal substance, reunite in a plexiform manner, and they have described this as their natural mode of termination. This opinion is in all probability founded on deceptive appearances, such as must often have presented themselves when the means of observation were less per fect than they now are, and which even at this time are but of too frequent occurrence. It appears to be a general fact that the tubes divide in their course from the apices of the medullary COneS tozvards their opposite termina tions, but they never reunite while passing in this direction. Other anatomists have con sidered the tubes to terminate in free blind extremities unconnected with the Malpighian bodies ; and they have based their opinion on the appearances of injected specimens as well as on those of recent ones. With reference to this question Mr. Bowman* remarks, " As the in,jection always stops short of the real extremities of the tubes, it must necessarily show apparent free extremities—and others may be produced by the section requisite for the examination of the part. As for the false appearances presented by recent specimens, they are obviously referable to the sudden bending down of a tube behind the part turned to the observer. In a mass composed of convolutions, many such must continually occur ; and their real nature may be easily determined by the use of a high power and varying focus." In addition to the sources of fallacy thus alluded to by Mr. Bowman, there is another, to which I have already referredt in describing the fibrous matrix in which the tubes are packed. To an inexperienced obser ver, few appearances could be more deceptive than the apparent abrupt terminations of the tubes, as these are seen in the spaces formed by the surrounding tissue, here visible in the meshes of the network, and there suddenly concealed as they pass beneath the fibrous tissue.