With the inconsiderable exceptions above mentioned, the terminal twigs of the artery correspond in number with the Malpighian bodies. Arrived here, the twig, which is usually of considerable length, although occasionally very short, perforates the capsule, and, dilating suddenly, breaks up into two, three, four, or even eight branches, which diverge in all directions like petals from the stalk of a flower, and usually run in a more or less tortuous manner, subdividing again once or twice as they advance over the surface of the ball they are about to fortn. (Fig. 153.) They dip into its interior at different points, and, after further twisting. reunite into a single small vessel (fig. 154. 1.), which varies in its size, being generally smaller, but in some situations larger, than the terminal twig of the artery. This vessel emerges between two of the primary divisions of the terminal twig of the artery, perforating the capsule close to the vessel, and, like it, adhering to the membrane as it passes through. It then enters the capillary plexus, which surrounds the tortuous uriniferous tubes. (Fig. 154. 2. and 155.) The The vessels resulting from these subdivisions are capillary in size, and consist of a simple, homogeneous, and transparent membrane.
tuft of vessels thus formed is a compact ball, the several parts of which are held together solely by their mutual interlacement, for there is no other tissue admitted into the capsule besides blood-vessels. It is subdivided into as many lobes as there are primary branches of the terminal twig or efferent vessel, and these lobes do not communicate, except at the wards upon tbe trunks of the vessels, if this term can be correctly applied to such minute root of the tuft. There are, therefore, deep clefts between them, which open when the lobes are not greatly disteqed with injection or blood. (Fig. 153, 10 t The surface of the tuft is everywhere unattached and free, and continuous with the opposed surfaces of the lobes. The whole circumference of every vessel composing the tuft is also free, and lies loose in the cavity of the capsule. These circumstances cannot be seen in specimens gorged with injection, but only by careful examination of recent specimens with a power of 200 or 600 diameters. _ The vessels are so perfectly bare, that in no other situation in the body do the capillaries admit of being so satisfactorily studied. It is only where the tuft is large, as in man and in the horse, that its lobulated character can be always discerned. Where the number of pri mary subdivisions of the afferent vessel is smaller, the detection of lobes is less easy ; they may often be seen, however, in the frog. In Birds and Reptiles the afferent vessel seldom divides; but dilates instead into a pouch-like cavity, which, after taking two or three coils, contracts again, and becomes the efferent vessel. There are of course no lobes ;
but the surface of the whole dilated part is free.
The basement membrane of the uriniferous tube, expanded over the Malpighian tuft to form its capsule, is a simple homogeneous and transparent membrane, in which no structure can be discovered. /t is perforated, as before stated, by the afferent and efferent vessels, and is certainly not reflected over the2n. They are united to it at their point of transit, but in what precise manner Mr. Bowman has not been able to determine. It appears probable that the membrane is reflected to a slight ex tent upon the afferent and efferent vessels, and that thus the union is effected. The appearance of bulging presented by the dis tended capsule round the entrance of the afferent vessels in fig.. 156. seems to indicate that the inembrane is slightly reflected in vessels as the afferent and efferent vessels of the Malpighian bodies. Opposite to the point where the vessels perforate the capsule, is the orifice of the tube, the cavity of which is con tinuous with that of the capsule, generally by a constricted neck. Mr. Bowman has speci mens prepared with the double injection, showing this continuity in Mammalia, Birds, Reptiles, and Fish. (fig. 157.) A more satisfactory proof of the direct continuity of the cavity of the tube with that of the Malpighian capsule is afforded by a clear view of the whole of the textures mag nified 200 or 300 diameters. The capsule may thus be seen to pass off into the basement membrane of the tube as the body of a Flo rence flask into its neck (figs. 153. and 159.).
The basement membrane of the tube is lined by a nucleated epithelium of a finely granular opaque aspect, while the neck of the tube and the contiguous portion of the cap sule are covered by a layer of cells much xnore transparent, and clothed with vibratile cilia. The epithelium appears to be continued in many cases over the whole inner surface of the capsule, while in other instances it is impossible to detect the slightest appearance of it over niore than a third of the capsule. When fairly within the capsule the cilia cease, and the epithelium beyond is of excessive delicacy and translucence. Its particles are seldom nucleated, and appear liable to swell by the application of water to the specimen. The cavity existing in the natural state be tween the capsule and the vascular tuft, is filled by fluid, in which the vessels are bathed, and which is continually being impelled from the capsule into the tube by the lashing move ment of the cilia.