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OP SECRETION AND EXCRETION.
SECTION III.
OF SECRETION.
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CHAPTER I.
Of the Secretions in General.
During the passage of the blood from the minute arteries through
the capillary system of vessels into the radicles of the venous system,
a part of the “liquor sanguinis,;55 with the matters dissolved in it, is
imbibed by the tissues, and by their agency undergoes a chemical
change. Some of its components are extracted from it, while it re¬
ceives in exchange other matters derived from the parenchyma.
The changes which the organic matter suffers in this way may be
termed generally transformations or “ metamorphoses.55 They are
of three kinds:—
1. Transformation of the components of the blood into the or¬
ganised substance of the different organs,—“ intus-susceptio ” or
nutrition, which will be treated of in the next section.
2. Transformation of the components of the blood on the free sur¬
face of an organ into a solid unorganised substance, which is the
mode of growth of the non-vascular textures,—“ apposition
We have, however, already seen, that the substance composing
the non-vascular tissues is as much organised as the elementary parts
of the vascular structures, and that their process of growth is essen¬
tially the same.
3. Transformation of the components of the blood into a fluid mat¬
ter which escapes on the free surface of the organ,—secretion, which
is the subject of the present section.
The matters separated from the blood by the action of a secreting
organ are,—1. Substances which existed previously in the blood, and
are merely eliminated from it: such are the urea, which is excreted
by the kidneys; and the lactic acid and its salts, which are compo¬
nents both of the urine and of the cutaneous perspiration. These
are called excretions; and the process of their separation from the
blood, excretion. The excretions which are met with most gene¬
rally in the animal kingdom, namely, the urine, and the fluid perspired
by the skin, are in the human subject acid; but all excretions are not
acid, as Berzelius formerly supposed, for the urine of some herbivo¬
rous animals is alkaline, as are also some of the excretions peculiar
to several animals; for instance, the acrid matter excreted from the
skin of the toad. 2. Substances which cannot be simply separated
from the blood, since they do not pre-exist in it, but which, on the
contrary, are newly produced from the proximate components of the
blood by a chemical process; such are the bile, the semen, the milk,
mucus, &c. These are called secretions.