ESSAY ON SECRETION. 
9 
Liebig also states that if bile be injected into the rectum, 
it will be absorbed, and cannot afterwards be detected in 
the urine, which, as he thinks, supports the theory that it 
becomes changed in the blood. 
Leaving for the present the consideration of this question, 
I will now direct your attention to one of very considerable 
importance as connected with the physiology of secretion; 
namely, the inquiry, Are all the secretions pre-existent in 
the blood, or are they formed therefrom by the glands ? 
In order to the giving a definite answer to this question, 
it will be necessary to investigate, first of all, the general 
structure of the glands themselves; then the composition of 
their secretions; and, further than this, to consider some 
pathological facts which will materially assist us in arriving 
at its correct solution. 
Without the aid of diagrams, it will, perhaps, be difficult 
to give you a clear idea of the structure of glands, but you 
will find illustrative drawings in almost all the works that l 
have named; and I may fairly expect that all of you are, 
more or less, acquainted with this peculiarity. 
In the mucous and synovial membranes, the secretion is 
formed by the layer of cells which covers their free surfaces; 
which cells, as they grow, draw from the blood the liquid con¬ 
stituting the secretion of the membrane, and either by rupture 
or by exudation, pour it out upon the surface of the mem¬ 
brane. In certain situations, as the villous coat of the sto¬ 
mach, and part of the intestinal canal, we find depressions 
within the mucous membrane, lined by cells which separate 
certain matters from the blood, as the gastric fluid in the sto¬ 
mach, &c. These may be considered as rudimentary glands; 
for, in order to the construction of a gland, all that is neces¬ 
sary is that this follicle shall become elongated and increased 
in calibre, branched at one end, and covered by a fibrous 
tissue corresponding to the corium of the mucous membrane. 
Each of the branches of this elongated follicle, now called a 
gland duct, and often containing muscular tissue among the 
fibres of its external coat, terminates in one or more caecal 
extremities, wffiich either may be dilated so as to assume the 
appearance of little vesicles, or may be convoluted without 
dilatation. 
The cells which separate from the blood the special secre¬ 
tion of the gland, are situated either in the vesicles in w 7 hich 
the branches of the duct terminate, or in the convoluted por¬ 
tions of the duct itself. They draw their materials from the 
blood, contained in a vascular plexus, which is situated be- 
xxiii. 2 
