arteries and veins of a section of the skin.
A, A, Arterial branches. B, B, Capillary, or hair-like vessels, in which
the large branches terminate. C, The venous trunk, collecting the blood
from the capillaries.]

625. The ARTERIES AND VEINS of the skin are very numerous. The larger
branches of the arteries pass through the open meshes of the true
skin, and are subdivided into a myriad of minute capillary vessels,
which form a beautiful net-work on the upper surface of the true skin.
This vascular net sends a branch to each of the papillae, which opens
into and terminates in a minute vein. The capillary veins are as
numerous as the arteries which they accompany. They unite and form
larger trunks, as small springs from the hill side coalesce to form
rivulets.

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624. What vessels are found in the cutis vera? Explain fig. 112. 625.
What is said of the cutaneous arteries? Of the cutaneous veins?

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626. The NERVES that are spread over every part of the sensitive layer
of the true skin, proceed from the spinal cord. As a proof of the
great number of nervous filaments in the skin, no part of this tissue
can be punctured with a fine needle without transfixing a nerve, and
inducing pain. In some parts of the system, however, the nerves are
more abundant than in others; where the sense of feeling is most
acute, we find the greatest number of nerves, and those of the largest
size. Those parts that are most exposed to injury are most sensitive.

_Examples._ 1st. The conjunctiva, or skin of the eye, is pained by the
presence of a particle of dust, because it would render vision
imperfect.

2d. The lungs, also, would be injured by the smallest particle of
matter; they are therefore protected by the exquisite sensitiveness of
the lining membrane of the trachea, so that a particle of food or dust
is ejected by a convulsive cough before it reaches the lungs.

627. The nerves are more numerous in the upper than lower extremities;
in greater numbers upon the palm than the back of the hand. They are,
likewise, more abundant and larger at the extremities of the fingers,
and in the lips, than in any other part of the skin.

_Observation._ The proboscis of the elephant, the extremities of the
tails of certain species of monkeys, and the tentacula of some kinds
of fish, receive a more abundant supply of sensitive nerves than other
parts of their systems.

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