What is the structure of the capillaries? So great is their tenuity
that we evidently cannot have upon this point, any kind of data founded
upon experiment and observation. Only it is very probable, it is even
certain, that this structure is modified differently in each organ,
that it is not the same in the tendons, the aponeuroses, the muscles,
&c. that it really partakes of the nature of the organ of which it
makes an integral part.
The membrane which lines the excretories, the arteries, the veins,
the exhalants, vessels which go into the system of the capillaries or
come out of it, is very like that of these capillaries; but it is not
certainly the same.
It is the diversity in the structure of the capillaries, according to
the organs in which they are found, which has an essential influence
upon the difference which the vital properties exhibit, particularly
the organic sensibility and the insensible organic contractility in
each system in which we examine them; hence peculiar modifications in
all those diseases over which these properties preside, and which are
seated especially in the capillaries, such as inflammations, tumours,
hemorrhages, &c. &c.
The difference in structure of the capillary system, sometimes becomes
manifest to the eye. Thus the spleen, the corpus cavernosum, instead of
presenting, like the serous surfaces, a vascular net-work in which the
blood oscillates in different directions, according to the motion it
receives, exhibit only spongy, cancellated textures, whose nature is
but little known, in which the blood appears often to stagnate, instead
of moving, &c.