Having thus briefly alluded to certain parts of Mr. Bowman's description of the Malpig hian bodies, the correctness of vvhich has been questioned, and having shown, as I hope, that only in one minute part of his clear and accurate account of their structure is any mo dification required, we may proceed to trace the blood-vessels in their course from the Malpighian bodies.
The blood, leaving theMalpighian tufts, is conveyed by their efferent vessels to the great renal reservoir, the capillary plexus surround ing the uriniferous tubes (figs. 152. 154. and 155.). The vessels lie in the interstices of the tubes, and everywhere anastomose freely, so that throughout the whole organ they con stitute one continuous network, lying on the outside of the tubes, in the substance of the matrix, and in contact with the basement membrane. This plexus is intermediate be tween the efferent vessels of the Malpighian bodies and the veins.
The efferent vessels of the Malpighian bodies are always solitary, and never inos culate with one another : each one is an iso lated channel between its Malpighian tuft and the plexus surrounding the tubes. They are formed by the union of the capillary vessels of the tuft, and emerge from its inte rior in the manner already explained. After a course of variable length they open into the plexus. Their size is various. In general they are smaller than the terminal twig of the artery, and scarcely, if at all, larger than the vessels of the plexus into which they discharge themselves. But where the Malpighian tuft is larger, the efferent vessel is usually large also, and divides into branches before en tering the plexus. This is eminently the case with those situated near the base of the me dullary cones, where the medullary and cor tical portions of the organ seem to blend. The efferent vessels from these large Mal pighian bodies are often three or four times the diameter of those of the plexus, and take a course towards the pelvis of the kidney between the uriniferous tubes (A. 154. I.). They were formerly mistaken for tubes. They branch again and again in the manner of arteries, and form the plexus with long meshes, which invests this part of the tubes.
Some of the veins springing from this plexus form the well-known network on the nipple shaped extremities of the cones, around the orifices ; and thence take, with the remainder, a backward course, likewise parallel to- the tubes, to empty themselves into various branches that he about the bases of the cones. The arrangement of the venous radicles on the cortex and on the surface of the kidney has been already described (fig. 145). The veins from the capsule and surrounding fat join the renal vein in some part of its course. It is probable that the capillaries of the vasa vasorum, within the substance of the organ, pour their blood into the capillary plexus sur rounding the tubes, as those of the hepatic artery do into the portal hepatic plexus of the lobules of the liver.
Thus, there are in the kidney two perfectly distinct systems of capillary vessels, through both of which the blood passes in its course from the arteries into the veins : the first, that inserted into the dilated extremities of the uriniferous tubes, and in immediate con nection with the arteries ; the second, that enveloping the convolutions of the tubes, and communicating directly with the veins. The former, which may be called the Malpighian capillary system, is made up of as many parts as there are Malpighian bodies. These parts are entirely isolated from one another; and as there is no inosculation between the arterial branches supplying them, the blood enters each in a direct stream from the main trunk. Each separate part also of this system has but one afferent and one efferent channel, and both of these are exceedingly small, compared with the united capacity of the capillary tuft. The artery in dividing dilates ; then follow branches which often exceed it in size, and which gradually break up into the finest. The efferent vessel does not usually even equal the afferent, and in size is often itself a capillary. Hence would arise a greater retard ation of blood in the tuft than occurs pro bably in any other part of the vascular system ; a delay that must be increased by the tor tuosity of the channels to be traversed.