Bauhaus-Universität Weimar

418 
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.
        

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