The skin, or integument, is the body's largest organ (fig. 6.1). In adults, it covers an area of 1.5 to 2.0 m2 and accounts for about 15% of the body weight. It consists of two layers: (1) a stratified squamous epithelium called the epidermis and (2) a deeper connective tissue layer called the dermis. Below the skin is another connective tissue layer, the hypodermis, which is also discussed in this chapter.
Most of the skin is 1 to 2 mm thick—about half as thick as the cover of this book. It ranges, however, from less than 0.5 mm on the eyelids to 6 mm between the shoulder blades. This difference is due mainly to variation in the thickness of the dermis. However, skin is classified as thick or thin skin based on the relative thickness of the epidermis alone, especially the surface layer of dead cells called the stratum corneum. Thick skin covers the palms, soles, and corresponding surfaces of the fingers and toes. It has an epidermis that is 400 to 600 ^m thick, due to a very thick, tough stratum corneum (fig. 6.2b). Thick skin has sweat glands but no hair follicles or sebaceous (oil) glands. The rest of the body is covered with thin skin, which has an epidermis 75 to 150 ^m thick, with a thin stratum corneum (see fig. 6.3). It possesses hair follicles, sebaceous glands, and sweat glands.
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.