Nails And Glands

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There are two important groups of skin accessory structures we have not yet mentioned. These are the nails and two types of glands in the skin.

A nail as a slab of keratin

Take a glance down at the surfaces of your digits (DIJ-its). Although digits are literally ''fingers'' in Latin, the word also describes your toes. A nail is basically a thick slab of stratum corneum (COR-nee-um) - the hard, ''horny'' (corne) outermost ''layer'' of the epidermis (see Figure 6.2).

Lunula-

Lunula-

Hair shaft

-Sweat pores -Sweat duct Sweat gland

Fig. 6.2 Nails and glands: Helpful ''things added'' to the skin. (A) Dorsal aspect of the thumb. (B) Sweat and sebaceous glands near hair follicles.

Hair shaft

-Sweat pores -Sweat duct Sweat gland

Fig. 6.2 Nails and glands: Helpful ''things added'' to the skin. (A) Dorsal aspect of the thumb. (B) Sweat and sebaceous glands near hair follicles.

The nail is hard and stiff because it consists essentially of thousands of dead squamae - keratin-stuffed scales - tightly pressed together into a flat bed or sheet. For all practical purposes, therefore, a nail is a tough, horn-like slab of keratin.

The body of the nail is the main, pinkish-colored, rectangle-shaped portion you can see on each of your digits. The pink color is created by a network of blood capillaries (CAP-ih-lair-eez), lying underneath. The capillaries are like pinkish ''little hairs'' (capill) filled with reddish-colored blood.

The pinkish body is bordered anteriorly and posteriorly (front-and-back) by two whitish areas. These are the free edge in front, and the lunula (LOON-you-lah) in the back. Both look whitish, rather than pinkish. The free edge is the curved portion extending beyond the digit. It is white due to a lack of any underlying blood capillaries. The lunula resembles a ''little moon'' (lunul) shaped like a whitish crescent. There are blood capillaries under the lunula, but the bottom layers of the epidermis are too thick in the lunula to allow us to see any pink.

CHAPTER 6 Anatomy of the Skin ''Don't sweat it! - Just get greasy!''

In addition to the nails, there are several types of glands found within the skin. Chapter 3 defined glands, and noted that there are two main varieties -exocrine glands and endocrine glands. The skin is rich in two types of exocrine glands, which are glands of external secretion of some useful product into a passageway or duct.

Sweat glands are technically called sudoriferous (soo-dor-IF-er-us) glands, because they are ''sweat'' (sudor) ''carriers or bearers'' (fer). There are about 3 million sweat (sudoriferous) glands scattered throughout the dermis (see Figure 6.2,B). Each sweat gland has a highly coiled body, which secretes sweat into a long sweat duct. Finally, the sweat duct empties into a sweat pore on the skin surface. Sweat is rich in water and sodium chloride (NaCl), and also contains small amounts of waste products, such as urea (you-REE-uh) and lactic (LAK-tik) acid. Sweat plays an essential role in body cooling and thermoregulation (THER-moh-reg-you-LAY-shun). By thermoregulation, we mean the ''regulation'' or control of body ''heat'' (therm) or temperature. The specific mechanisms of thermoregulation (control of body temperature) are discussed in some detail within our companion volume, PHYSIOLOGY DEMYSTIFIED.

Sebaceous (sih-BAY-shus) glands ''involve or pertain to'' (-ous) ''grease'' (sebac). The great majority of the several million sebaceous glands within the dermis are attached to the sides of hair follicles (see Figure 6.2,B). The sebaceous glands continually produce and secrete sebum (SEE-bum) or skin ''grease'' (seb). Sebum plays an often-underappreciated role in lubricating the hairs and skin surface. (Think about what happens to the skin on the hands of many people, such as nurses, who have to wash their hands often. Especially in the winter, not having enough sebum results in dry, red, painfully cracked skin!)

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