74 
MR. BOWMAN ON THE STRUCTURE AND USE OF 
pands to embrace the tuft of vessels. From being opake and minutely mottled, it 
becomes transparent, and assumes a definite outline. From being bald, it becomes 
covered with cilia (at least in reptiles, and probably in all classes) ; and, in many 
cases, it appears to cease entirely, a short way within the neck of the Malpighian 
capsule. 3. The blood-vessels, instead of being on the deep surface of the membrane, 
pass through it, and form a tuft on its free surface. Instead of the free anastomosis 
elsewhere observed, neighbouring tufts never communicate, and even the branchlets 
of the same tuft remain quite isolated from one another. 
Thus the Malpighian bodies are as unlike, as the tubes passing from them are like, 
the membrane, which, in other glands, secerns its several characteristic products 
from the blood. To these bodies, therefore, some other and distinct function is with 
the highest probability to be attributed. 
When the Malpighian bodies were considered merely as convoluted vessels without 
any connection with the uriniferous tubes, no other office could be assigned them, 
than that of delaying the blood in its course to the capillaries of the tubes, and the 
object of this it was impossible to ascertain. Now, however, that it is proved that 
each one is situated at the remotest extremity of a tube, that the tufts of vessels are 
a distinct system of capillaries inserted into the interior of the tube, surrounded by a 
capsule, formed by its membrane and closed everywhere except at the orifice of the 
tube, it is evident that conjectures on their use may be framed with greater plausibility. 
The peculiar arrangement of the vessels in the Malpighian tufts is clearly designed 
to produce a retardation in the flow of the blood through them. And the insertion 
of the tuft into the extremity of the tube, is a plain indication that this delay is sub- 
servient in a direct manner to some part of the secretive process. 
It now becomes interesting to inquire, in what respect the secretion of the kidney 
differs from that of all other glands, that so anomalous an apparatus should be ap- 
pended to its secerning tubes ? The difference seems obviously to lie in the quantity 
of aqueous particles contained in it ; for how peculiar soever to the kidney the 
proximate principles of the urine may be, they are not more so than those of other 
glands to the organs which furnish them. 
This abundance of water is apparently intended to serve chiefly as a menstruum 
for the proximate principles and salts which this secretion contains, and which, 
speaking generally, are far less soluble than those of any other animal product. 
This is so true, that it is common for healthy urine to deposit some part of its dis- 
solved contents on cooling. It may seem that an exception to this exists in the solid 
urine of some reptiles; but this expression merely describes the urine as it is found 
in the cloaca and larger excretory channels. The secretion is brought from the 
tubules of the gland in a fluid state, and only becomes solid by the re-absorption of 
its aqueous portion after it has traversed the tortuous canals wherein it was formed, 
and been placed in a condition to be readily expelled from the system. The subor- 
dination of the aqueous part to the purpose of eliminating the more essential elements 
