THE MALPIGHIAN BODIES OF THE KIDNEY. 
63 
strictly speaking-, an involution of the outer tegument of the frame : their interior 
is, in one sense, the outside of the body : their walls intervene between the vessels 
and the exterior, and, as it were, cover them in. But here is a tuft of capillaries 
extruded through the wall of the tube, and lodged in a dilatation of its cavity, un- 
covered by any structure. Bare indeed, yet screened from injury in its remote cell, 
with infinite care and skill ! 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 must arise a greater retardation of the blood in the 
tuft, than occurs probably in any other part of the vascular system ; a delay that 
must be increased by the tortuosity of the channels to be traversed. 
The other system of capillaries, or that surrounding the uriniferous tubes, corre- 
sponds, in every important respect, with that investing the secreting canals of other 
glands. It is well known to anatomists, and therefore does not require to be de- 
scribed at any length. Its vessels anastomose with the utmost freedom on every side, 
and lie on the deep surface of the membrane that furnishes the secretion. 
I have applied the term { portal system of the kidney ’ to the series of vessels con- 
necting these two, on account of the close analogy it seems to bear to the vena porta. 
The precise quality of the blood it carries may be doubtful, but in distribution it is 
similar. It intervenes between two capillary networks, the first of which answers 
to that in which the vena porta originates, and the second to that in which the 
vena porta terminates. The obvious difference lies in its several parts not uniting 
into a single trunk, to subdivide afterwards; but this circumstance seems to admit 
of an easy explanation. A trunk is formed in the great portal circulation, for the 
convenience of transport, most of the capillaries which supply it lying at a distance 
from the liver. Some, however, viz. those drawn from the hepatic artery, either enter 
the portal-hepatic plexus directly, (as Muller thinks, and as my preparations cer- 
tainly show some of them to do,) or else join the minuter twigs of the portal vein, 
according to the opinion of Kiernan. Now, in the kidney, the vessels issuing from 
the Malpighian tufts are disseminated pretty equally throughout the plexus sur- 
rounding the tubes (the one into which they have to discharge themselves), and they 
therefore enter it at all points at once, without uniting. In the medullary cones, 
however, where there is a capillary plexus to be supplied with blood, but no Malpig- 
hian bodies nearer than the base of the cones, the conditions which oblige the form- 
ation of a portal venous trunk begin to operate ; the two capillary systems it serves 
to connect are at some distance apart. Here, consequently, the Malpighian bodies 
are generally larger, their efferent vessels more capacious, and branched after the 
manner of an artery. Each one of these efferent vessels is truly a portal vein in mi- 
niature. 
