76 
MR. BOWMAN ON THE STRUCTURE AND USE OF 
eliminated: and all secretions designed for an ulterior use in the oeconomy must be 
assimilated by such a tissue in order to their separation from the blood. This tissue 
is the epithelium of such surfaces, as, from their external anatomical position, can at 
once release the secretion, when its elaboration is accomplished. The epidermis of 
the skin, the epithelium of mucous membranes, and that of true glands, all more or 
less completely fulfil this purpose ; but the first is chiefly designed as a protection, 
the second partly so, and the third is the only one entirely devoted to what is properly 
called secretion. Into the examination of this general question, it is impossible that 
I should now enter, but I shall state some considerations connected with it, that 
seem to have a bearing on the present subject. 
This theory, in its widest sense, supposes the epithelium of secreting surfaces either 
to pass through constant stages of renovation and decay, or else to remain, during a 
longer period, as a permanent organic form, assimilating and rejecting, in the mode 
just described. In many cases the epithelial particles appear to be cast off entire 
when their growth is complete, and thus to form the secretion ; in other instances, 
they seem to lose their substance by a more gradual process, and to waste or dissolve 
away on the surface of the membrane, as fresh particles are deposited below; in 
other examples still, there is reason for believing that they are long a persistent 
structure. It supposes that the elements of all natural secretions have at one time 
been a part of an organized form, the epithelial particle ; but it leaves it uncertain, 
whether the secretion, in a complete state, always exists in such particles when alive. 
It does not determine whether the chemical changes which occur in such particles, 
issue in the completion of the secreted product, until the period arrives for its being 
shed from the body. Hence it is beyond the reach of objections founded on the 
chemical examination of glandular organs en masse. 
Applying this theory to the kidney, it may be considered highly probable that the 
epithelium of the uriniferous tubes is continually giving up its effete particles, and 
undergoing a gradual decay. This view harmonizes in a striking manner with what 
has been before advanced as to the use of the Malpighian bodies. If the peculiar 
urinous principles were poured out at once, through the walls of the tubes by the 
capillaries surrounding them, they must be in a dissolved state from the first, and 
could need no further aqueous current to carry them off ; but if they are deposited 
in a more or less solid form, as a part of an organized tissue, they will require (being 
so sparingly soluble) an additional and extraneous source of water, by which, 
when their formation is complete, they may be taken up and conveyed from the 
gland. The correspondence before noticed (p. 72) between the size of the Mal- 
pighian bodies and the length of the tubes coming from them, is a strong argument 
in favour of this view. 
I stated that the large quantity of water in the urine seemed chiefly to serve the 
purpose of a menstruum. But though this quantity is always large, compared with 
that in other secretions, it is liable to great variation, according to the state of fulness 
