Nelsen, Olin E. Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania.
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A coiled, tubular gland found in mammals. There are two kinds, merocrine (or eccrine) and apocrine. The latter are generally associated with hair follicles (see illustration). Merocrine glands are distributed extensively over the body in the human, whereas the apocrine variety is restricted to the scalp, nipples, axilla, external auditory meatus, external genitals, and perianal areas. Apocrine sweat glands are more numerous in mammals, with the exception of the chimpanzee and human, in which the merocrine variety predominates. The mammary glands probably represent modified apocrine sweat glands which grow inward and increase in complexity. In association with adipose tissue, they eventually form pendant structures, the mammae, which project outward from the general contour of the skin's surface. The secretion process is apocrine with a considerable portion of the cell being discharged. The discharged portion of such a gland cell disintegrates to free fat droplets and albuminous substances. A mammary gland is complex and represents an association of lobes. Each lobe contains a compound alveolar (acinous) gland with a separate lactiferous duct which opens on the nipple in the human. The glands of Moll associated with the eyelashes are relatively large modified apocrine glands as are the ceruminous or wax glands in the external auditory meatus. The anal sacs of the skunk presumably are apocrine glands modified by the addition of muscle fibers from the levator ani muscle which enables the pungent contents to be ejected with force. See also: Epithelium; Gland; Lactation; Mammary gland
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