b. Sensory Innervations. If one looks at a zebra or a tiger, one can
immediately see that the fur of these animals has a belt-like color pattern. There is also
a belt-like pattern in the integument of humans. It is not a pattern of colors, as with
zebras and tigers. It is a pattern of sensory innervations. A "belt" is innervated by a
specific spinal nerve, left and right. This belt-like area is called a dermatome. We refer
to the nerves supplying these areas as segmental nerves because they "segment" the
integument into dermatomes. Except for the three dermatomes of the face, there is an
overlap of adjacent dermatomes.
3-22. INTEGUMENTARY MUSCLES
Also associated with the subcutaneous layer are a number of integumentary
muscles.
a. Facial Muscles. As the term implies, facial muscles are associated with the
face. Facial muscles are mainly involved with the various openings of the face. They
are able to open and close these openings. Because they are also used in visual
communication, they are sometimes called mimetic muscles ("muscles of expression").
b. Arrector Pili Muscles. Another group of integumentary muscles is known as
the arrector pili muscles. Ordinarily, the hairs and the hair follicles are at an angle to the
skin rather than perpendicular (straight up or down). At times of emotional stress, the
arrector pili muscles contract. In hairy areas, the contraction of these muscles, attached
to the follicles, causes the hairs to stand "straight up." In glabrous areas, their
contraction produces "goose bumps."
Section VI. INVESTING DEEP FASCIA
3-23. INTRODUCTION
The innermost of these three concentric layers is the investing deep fascia. The
investing deep fascia is essentially a membrane of dense FCT completely surrounding
the body. It overlies all of the remaining structures of the body.
3-24. VARIATIONS IN THICKNESS
a. The investing deep fascia varies in thickness in various parts of the body.
This membrane is generally thicker the further inferior we go. In many areas, it is thick
enough to be specifically named. For example, the investing deep fascia of the lower
member is called the fascia lata.
b. The majority of the tissues of the body are made up primarily of water.
Moreover, the interstitial spaces are filled with water. Therefore, the body within the
investing deep fascia can be thought of as a hydrostatic column. As such, hydrostatic
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