70 
MR. BOWMAN ON THE STRUCTURE AND USE OF 
will probably appear conclusive. Even of the testis (where the tubes are far thicker 
and stronger in their coats, and much more capacious than in the kidney), there are 
not ten specimens that can be pronounced at all full, in the museums of Europe ; and 
there is no evidence, that, even in the best of these, the injected material has reached 
the very extremities of the tubes. 
In the kidney, the tubes are exceedingly tortuous after leaving the Malpighian 
bodies, and only become straight, in most animals, in proceeding towards the excre- 
tory channel to discharge themselves. The way towards their orifices is so free, in a 
natural state, that their fluid contents exert no distending force upon their walls. 
Accordingly their walls are exceedingly feeble ; the basement membrane on which 
their strength mainly depends, is very delicate and easily torn. They are therefore 
incapable of offering much resistance to a fluid impelled into them from the pelvis, 
but burst readily, if it be forcibly urged. But were the coats ten times as tough as 
they really are, injection could not penetrate far into their convoluted portion, unless 
pushed with much force ; and this for two reasons : — 1st. The fluid which the tubes 
already contain has no means of escape before the injection, since these canals end by 
blind extremities in the Malpighian bodies ; and though these bodies are dilatations 
of them, yet they are already filled almost completely by the tuft of capillaries, and 
offer no capacious receptacle for the fluid from the tubes. 2nd. The layer of epi- 
thelium (which usually forms about two-thirds of the thickness of every tube, the 
calibre being about one-third*) is, immediately after death, very prone to separate 
from the basement membrane which it lines, and to fall into and block up its narrow 
channel. Even if the epithelium remains in its place, the calibre of the tube is but small, 
and if it becomes detached, it opposes an effectual bar to the progress of the injection. 
By removing the pressure of the atmosphere from the outer surface of the tubes, 
these obstacles are occasionally in part overcome, so that even the tortuosities of the 
tubes are filled for a certain distance. But even so limited a success is rare, and in 
face of mechanical obstacles, such as above mentioned, to the onward current of the 
injection in the tubes, the force employed invariably sooner or later bursts their 
coats, ere their extremities have been reached. Extravasation from the tubes, as might 
be expected, fills their interstices, and the fluid may then issue by a rent at the hilus 
of the kidney. But it is remarkable how readily it enters the veins and absorbents 
from the ducts. This is undoubtedly by extravasation, and does not prove any con- 
tinuity between them. The veins may be filled when the fluid has not penetrated in 
the tubes beyond the medullary cones, showing that the rupture must occur in con- 
nection with those cones, either at their apices or in their substance. By a thin 
transverse section of one of these cones, the ducts and blood-vessels of which they 
principally consist, are seen to be imbedded in a sort of matrix, apparently homoge- 
neous, but probably having a cellular structure. This matrix keeps the tubes and 
* These proportions vary considerably. The basement membrane is so thin that it may be left out of the 
estimate. 
