12 
ESSAY ON SECKETION. 
it may either enter the gall-bladder, in those animals that are 
furnished with such an appendage to the duct, or pass directly 
onward into the intestine, as in the horse. 
I must now say something of the secreting structure of the 
kidneys. 
You are all familiar with the division of each of these glands 
into a central, or medullary, and an external, or cortical por¬ 
tion, as well as with the dilated commencement of their ex¬ 
cretory duct, known as the pelvis of the kidney; so that I 
need give no anatomical details respecting those parts. I 
may therefore at once proceed to the description of the cor¬ 
tical or secreting portion of the gland. This contains a great 
number of little globular bodies known as Malpighian tufts, 
each of which has, in the horse, a diameter of from one nine¬ 
tieth to one fifty-fifth of an inch ; and is composed of a thin 
capsule enclosing a tuft of capillary bloodvessels, which cap¬ 
sule communicates with the origin of a little tube arising 
therefrom. These little tubes are at first very convoluted, 
and are lined by a spheroidal epithelium, and covered on their 
outer surface by a capillary plexus of bloodvessels, respecting 
which we have to observe, that it is not formed by the splitting 
up of small arteries, but by the subdivision of small veins 
which receive their blood from the plexus within the Mal¬ 
pighian tufts. 
Investigations into the comparative structure of the urinary 
organs in various classes of animals, have led to the conclusion 
that the spheroidal epithelium, contained in' the little convo¬ 
luted tubes, is the true secreting structure of the kidney, the 
tufts serving only for the drawing from the blood the watery 
portions of the urine. 
In the medullary portion of the kidney the tubes take a 
straight course, and gradually converging, form by junction 
one with another, those cone-shaped or pyramidal bodies 
which have been called the pyramids of Malpighi. They at 
last terminate by conical projections into the pelvis of the 
kidney, and are designated its mamillary processes. They 
are there surrounded by folds of the mucous lining membrane 
of the pelvis, called calyces; the spaces between the projecting 
mamillary processes constituting the infundibula of the kidney. 
I have already spoken of the composition of some of the 
simpler secretions, and shall now enter upon the constitution 
of the two secretions, bile and urine. 
These are both complex in their character. The others, it 
will be remembered, consisted principally of water, a few salts, 
and a principle very similar to the albumen of the blood. 
This, too, so far as chemistry is concerned, is nearly all that 
