In the remaining portions of the tubes which intervene between the neck of the Malpighian capsules and the bases of the medullary cones, the epithelium presents itself under the form to which the term spheroidal or glandular is commonly applied.* The particles are of a more or less rounded form, and are thus distinguished from the flattened cells of the lamelliform or scaly variety of epithelium. (Fig,. 163.) They usually form a single lay er covering the surface of the basement mem brane. They are granular and opaque, and appear to contain a considerable quantity of solid matter. The cell wall is very delicate, and when water is added to the specimen, the cells frequently fall in pieces very rapidly. In this respect the cells of the kidney differ re markably from the hepatic cells, the latter having a much thicker and firmer wall, which offers a greater resistance to the action of water. The cells have a distinct nucleus, and in the centre of this in many instances a nucleolus is clearly visible. (Fig. 164.) Another interesting feature in the renal se creting cells consists in their containing in some cases minute particles of oil. In a perfectly healthy kidney, the quantity of oil contained in the epithelium is very small ; sometimes, indeed, it is difficult to find any cells which contain even the most minute particles of oil, while in other instances, where there is every reason to consider the organ quite healthy, the quantity of oil is much more considerable. When this material accumu lates beyond a certain extent which it is diffi cult to define, it must be considered as morbid, and a great excess of oil in the secreting cells constitutes a main feature of one of the most serious and intractable diseases to which the kidney is liable.
The epithelium lining the straight tubes of the pyramids differs essentially from that of the convoluted tubes ; the latter, as before stated, is the true spheroidal or glandular variety of epithelium ; while the former approaches more nearly to the lamelliform or scaly variety. Its particles are smaller and more flattened, so that the epithelium in the medullary cones constitutes a much smaller proportion of the thickness of the tubes than does that in the convoluted tubes of the cortex. (Fig. 163. b.) The canal of the tubes in the medullary cones is also greater in proportion to the thickness of the wall than in the convoluted tubes. The cells in this portion of the tubes have uniform, smooth, and transparent walls, and their interior is less opaque and granular than is the case with the glandular cells before described. Another distinctive character con sists in the fact of these cells seldom, if ever, containing oil.
Ciliary motion in the tubes. — The preceding description of the epithelial lining of the uriniferous tubes corresponds in most par ticulars with the usually received account of these structures. There now remain to be stated certain facts which probably are not generally known even to those who are ac customed to make microscopical examinations of the kidney. In 1815, A. Kffiliker published a short paper*, in vvhich he mentions the interesting fact, that in the kidney of the em bryo lizard the uriniferous tubes are lined by an epithelium remarkable for distinctly deve loped ciliary processes, which may be seen in vigorous action for some time after the death of the animal. The ciliated epithelium, according to Kolliker's observation, exists throughout the whole length of the tubes, except at the extremities next the common excretory duct. He also observed the cilia
at the entrance of the Malpighian capsule. In a note appended to the same paper, the editor (J. Miiller) states that he has observed the same phenomenon in the uriniferous tubes of a fish (Raia clavata). The cilia are very large and long ; they are directed alow, the axis of the tube, and have a wavy motionblike that of a whip-lash.
In the spring of the present year, before I was aware of the observations just now referred to, while examining the kidney of the newt (Triton and Lissotriton), I was surprised to find vibratile cilia in active mo tion, not only within the Malpighian cap sule as described by Mr. Bowman in the frog, but apparently' extending throughout the whole length of the uriniferous tubes. I have since looked for this wonderful phenomenon in many of the animals just now mentioned, and have never failed to detect it in any one of the kidneys examined. The part of the kidney most favourable for the examination of this ciliary motion is the anterior extremity, where it is very thin and transparent, so that after being cut away 'with sharp scissors it requires no further preparation for micro scopical examination. In a part thus pre pared, I have sometimes seen the cilia in rapid action throughout the whole length of every tube in the field of the microscope, and a more wonderful or beautiful sight can scarcely be imagined. The motion commences within the Malpighian capsule; the little par ticles floating in the liquid of the capsule are darted into the orifice of the tube with mar vellous precision, and thence they are directed onwards through the windings of the tube in a current of liquid, which is propelled with great regularity and speed. Much violence in tearing up the specimen for examination ap pears to arrest the motion ; and when water is added to the preparation, the epithelial particles swell and fill up the cavity of the tube, and so the motion is retarded. When the cilia are in slow motion, their form and the direction of their movement may easily be seen ; but when the motion has entirely ceased, I have never been able to see them distinctly, even with the best object glasses. The motionless cilia appear to collapse and fall upon the surface of the epithelium, and so become in visible. Since my attention was first directed to the phenomenon in question, I have had but little time to search for it in other ani mals ; but there appears reason to believe that it exists in most of the higher animals, and pro bably even in man. The result of my own observations may be thus briefly stated : — In the newt I have searched for ciliary motion in the tubes many times, and have never failed to find it in any kidney which I have examined. I have searched for it in the frog twice (i. e. in two individuals), and found the ciliary motion very distinct in a considerable portion of one tube. I have examined one snake, and observed the motion very distinctly throughout a large extent of several of the tubes, as well as in the Malpighian capsules. I have searched for the phenomenon in the kidneys of some of the smaller Mammalia, as, for instance, in the mouse and the rabbit, but hitherto without success. I am not aware that any other observations with reference to this subject have been published, but possibly there may be some with which I am not acquainted.