REN. 



249 



only in the absence of the basement mem- 

 brane between the vessels and secreting cells. 

 Dr. Gerlach's figure (fig. 160.) exhibits an 



Fig. 160. 



Malpighian tiift of capillaries covered with small 

 transparent nucleated cells. (After Gerlach.} 



appearance which every one must have seen 

 in the tuft of vessels extended from the cap- 

 sule, but which fails to establish the existence 

 of this epithelial investment of the tuft ; for 

 at the border of the figure the wall of the 

 capillaries is seen actually bare, as described 

 by Mr. Bowman. The fact, however, seems 

 to be, that there do exist, here and there, upon 

 the outside of the capillaries of the tuft, nu- 

 cleated particles, of an extremely delicate 

 nature, the nuclei sometimes lying isolated in 

 the fork of two vessels, and the substance of 

 the cell not expanding into a continuous co- 

 vering of the whole tuft. It is possible that 

 these nucleated particles may be rather the 

 nuclei belonging to the capillary wall, than a 

 modified representation of the epithelium of 

 the tube. It is at least certain that they lie 

 sparingly upon the individual vessels of the 

 tuft, and do not form a membranous invest- 

 ment of it as a whole. Mr. Bowman showed 

 me these particles, as I have now described 

 them, some years ago. Their existence does 

 not affect the substantial accuracy of his ac- 

 count of the anatomy of the tuft, nor his view 

 of its special share in the secretion of urine. 



Having thus briefly alluded to certain parts 

 of Mr. Bowman's description of the Malpig- 

 hian bodies, the correctness of which 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 (fig. 154. 1.). 

 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 lie 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 si/stems 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- 



