The surface of the cortical portion fre qu ently presents an appearance oflobules, some what similar to those of the liver. The form of these lobules varies considerably ; in some instances they are circular, but very com monly they have a pentagonal or hexagonal outline; they are usually about 1 of an inch in diameter Cfig. 144).
This lobulated appearance of the surface is produced by the venous radicles which are dispersed at nearly equal distances throughout the cortical substance, each receiving the blood, as will presently be shown, from the plexus surrounding the convoluted tubes. These radicles unite in an irregularly arbores cent figure, anastomose, and form the several branches of the renal vein. Those on the sur face have a tendency to converge towards a central vessel, which then dips into the interior to join the trunks of the renal vein. Thus are formed the stellated vessels, which are often very conspicuous in diseased speciniens, when there has been an impeded circulation through the veins of the kidney. Between the arms of these stelke the convoluted tubes are visible on the surface (fig. 145). Ferrein supposed each of these lobules to form the base of a pyramid, the apex of the same being at the extremity of a mamillary process, and he believed that such an elongated pyra mid might be traced continuously from one part to the other, the tissues radiating from a point in the mamillary process, through the cortical substance, to one of the lobular divisions on the surface. Mr. Bowman's preparations show that " each lobule contains many tortuous ducts with their capillaries, but the convolutions of any one duct are not confined to a single lobule." Hence it is ma nifest that there is no natural division of the renal substance corresponding with the sup posed " pyramids of Ferrein," and there appears no reason for retaining a name which is not expressive of any fact or definite idea.
The medullary substance or, as it is some times called, the tubular portion, is of a firmer texture and darker red colour than the cortical portion, and presents itself under the form of cones or pyramids, (pyramids of Malpighi,) the bases of which are directed towards the surface of the kidney and continuous with the cortical portions, while the apices, which are called nzamillary processes, or papillee, are free and directed towards the cavity of the pelvis. The number of these pyramids has been variously stated by different anatomists ; their number is not constant, but it is usually about twelve or fifteen in each kidney. Some of the pyramids are compound, being formed by the union of two, which have one com mon mamillary termination. The number of
mamillary processes is therefore less, by four or six, than that of the pyramids. The cut surface of each pyramid has a striated appearance, being composed of tubes which subdivide and radiate in passing from the apex towards the base, where they merge into the cortical substance. These tubes are com monly named the tubes of Bellini, that anato mist having been the first to show their true tubular character ; they are united by a firm network of fibrous tissue in the substance of which there are some large veins, which take, for the most part, a straight course between the tubes. No Malpighiau bodies exist in the medullary cones.
The capsule of the kidney is a firm, white, fibrous membrane adherent by its external sur face to the adipose tissuein which the kidney is imbedded, and connected by its internal face to the entire surface of the kidney. It sends numerous fibrous processes into the cortical substance, and small vessels pass from the sub stance of the kidney into the fibrous capsule. These connecting bands between the kidney and its capsule are easily torn when the cap sule is stripped from the surface of the kidney. At the hilum of the kidney the capsule be comes continuous below with the ureter and above with the fibrous layer of the pelvis ; at the same point the blood-vessels receive an investment from this fibrous membrane, which is continued upwards with them until they finally break up into minute branches about the bases of the pyramids.
Calyces, Infundibula, and caly ces are membranous or fibro-mucous cylinders which receive in their upper extremities the apices of the mamillary processes. Where the membrane is reflected over the apices of the cones, it is perforated by numerous orifices of the tubes of Bellini, from which a liquid may be seen to escape when pressure is applied to the cones. The calyces are less numerous than the rnamillary processes, two or three mamillary processes being occa sionally received into one calyx. The caly ces unite into three small tubes, one cor responding with each extremity and one with the central portion of the kidney ; these have somewhat of a funnel shape, and are called infundzbula. The infundibula soon unite to form the pelvis of the kidney, which is a mem branous reservoir of a flattened oval figure, terminating belowin the ureter. The pelvis and infundibula are usually surrounded by loose reticular and adipose tissue. The blood-ves sels of the kidney are placed in front of these parts. The fibrous and mucous coats of the ureter are continuous with those of the pelvis, infundibula, and calyces.