e the appearance of felt-cloth; but more
deeply, the pores become progressively larger, and, upon the lower
surface, have a diameter of about a line, or one twelfth of an inch.
This gives the under surface the appearance of a coarse web. The
strands of the under surface of the true skin are connected with the
fibrous web, in which the sub-cutaneous fat of the body is deposited;
while the upper surface gives support to the sensitive, or papillary
layer, which is bedded upon it.

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Give the experiment. 620. What is said of the thickness of the cuticle
in different parts of the body? 621. Describe the cutis vera. By what
name is it sometimes called? What is the appearance of the upper
surface of the cutis vera? Of the under surface?

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_Observation._ When the skins of animals are immersed in a strong
solution of oak or hemlock bark, a chemical union takes place between
the gelatin, of which the true skin is mostly composed, and the tannin
of the bark. By this process leather is formed, and its peculiar
markings are owing to the papillary layer.

[Illustration: Fig. 111. An ideal representation of the papillae. 1, 1,
The cutis vera. 2, 2, The papillary layer. 3, 3, The arteries of the
papillae. 4, 4, The veins of the papillae. 5, 5, The nerves of the papillae.]

622. The sensitive layer of the skin is thin, soft, uneven, pinkish in
hue, and composed of blood-vessels, which confer its various tints of
red; and of nerves, which give it the faculty of sensation. The
unevenness of this layer is produced by small, elongated, conical
prominences, called _Pa-pil'lae_.

623. Each PAPILLA is composed of a minute artery, vein, and nerve.
Some of the prominences are arranged in concentric ovals, as may be
seen on the ends of the fingers; others are more or less parallel, and
pursue a serpentine course; some suddenly diverge, and again reunite,
as may be seen in the palm of the hand. Papillae are found in every
part of the skin. Consequently, their number is very great.

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How is leather formed? 622. What is the appearance of the sensitive
layer? What causes the unevenness of this layer? Explain fig. 111.
623. Describe the papillae.

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624. The cutis vera contains not only _Arteries_, _Veins_, and
_Nerves_ but _Lymphatics_, _Oil-Glands_ and _Tubes_, and _Perspiratory
Glands_ and _Tubes_.

[Illustration: Fig. 112. The