These are divided into eccrine and apocrine sweat glands. The first, the common sweat glands, are the more numerous and are found all over the skin. They are defined eccrine due to their mechanism of secretion that produces an almost aqueous liquid. The apocrine sweat glands, on the other hand, are present only in certain regions of the body. Examples are the armpits, around the genitals, on eyelids, and in the external part of the ear canal, etc. The apocrine glands differ from the eccrine glands in various ways including: their secretion product, their distribution, their being limited to certain areas of the body, their localization deeper in the skin, their having their opening in the hair sheath, and their strict linkage of function with the sexual phases of life.
Eccrine sweat glands are innervated by sympathetic nerves, and sweating does not occur in the denervated areas of the skin. In rare cases, individuals with congenital defects may lack eccrine sweat glands and in hot climates must take special precautions to avoid excessive increases in their body temperature. The apocrine sweat glands are under double nerve control but their activity is not notably influenced by variations in temperature. Therefore, strictly speaking, they should not be classified as sweat glands.
Eccrine sweat glands are merocrinous glands in that they produce an aqueous solution of low density that does not appreciably cause disintegration of the epithelial cells. Their principle function is that of thermoregulation: sweating to allow the body to rid itself of excessive heat. These glands begin to function only at temperatures above 18-20DC, when normal mechanisms of thermoregulation become insufficient. The secretory capacity of these glands means that several litres of water can be lost from the body hourly, taking with it toxins and other substances. It is the evaporation of this water that brings about cooling of the body.
Another essential point is that, in combination with transpiration (perspiration insensibilis), sweat accounts for a large proportion of the water that remains on the skin, forming part of the hydrolipid film. Eccrine sweat has an odour between that of musk and urine and contains numerous amino acids and organic substances that, together with secreted salts, constitute the buffering system of the skin. It is, therefore, essential because sweat and sebum together constitute the hydrolipid film on the skin's surface, which is the indispensable protective covering keeping the skin in good condition and, consequently, allowing it to perform its many essential functions.
Eccrine glands are defined as simple tubular, being composed of a thin canal connected directly to the surface and extending into the dermis almost until the hypodermis, where it winds around itself. There are two recognisable parts to the gland: the first, the secretory segment, is the ball of the gland situated in the mid-dermis. Here the cells absorb liquid derived from the surrounding capillaries and deliver it into the lumen, the gland interior in the duct that leads to the excretory portion. In this part of the gland the composition of the liquid is modified due to reabsorption of mineral salts, potassium, and some of the sodium. Sweat is, in fact, salty, even though the body has at least in part attempted to extract the precious mineral salts from it. The sweat glands are, as mentioned earlier, richly innervated by the sympathetic nerve system. This explains why intense emotions can be accompanied by excessive sweating.
These glands have little physiological importance. Their importance is derived more from the negative effects oftheir product, from which unpleasant odours arise. The product of these glands is a secretion rich in organic substances including large amounts of protein and steroids. This is due to their mechanism of secretion that results in the loss of the upper part of the sudoral cells. The secretion is extremely dense and provides an excellent broth for bacteria, which break down the organic compounds so producing volatile pungent substances. These glands are present only in certain regions of the body: the armpits, the groin, and around the genitals. In other mammals these glands are important because they produce pheromones, substances that are at the base of olfactory communication between animals both in sexual attraction and in the demarcation of territory.
In their structure the apocrine glands are similar to the eccrine sweat glands in that they too are simple tubular-alveolar. The apocrine glands are also composed of a secretory segment in the form of a ball and an excretory duct. The apocrine sweat glands, like the sebaceous glands, are always associated with a hair follicle, although these glands open directly onto the skin surface in the same region as that described for the sebaceous glands. In the outer ear, in particular, these glands have become the ceruminous glands characterized by a dense yellowish secretion.
In addition to the nail lamina, the hard part generally identified as nail, nails are also composed of surrounding tissues. Anatomically, the nail is a complex of the skin composed of the following tissues:
The formation of the nail lamina begins in the matrix. Here the cells multiply and become keratinized in a manner analogous to that in the hair. Though keratinization is similar in the epidermis, the hair, and the nail, in each case it is stabilized by a different pattern of linking disulphide bonds, which determine the anatomical and structural differences between the skin and its annexes.
The hyponychium is a short portion of normal epidermis just below the lamina that defines the end of the fingertip and the beginning of the nail bed. The latter is below the nail lamina and is covered by a particular epidermis. The peronychium is the epidermis that surrounds the entire nail, bounding it with a deep furrow. The characteristic little membranes, or cuticles, that surround the nail continue below it in the form of the eponychium, which cannot be seen. Finally, the lunula is the portion of the matrix seen at the base of the nail; its whitish appearance is due to its distance from the nail bed.
The nails, though in man less important than the claws of animals, still playa fundamental role both in protection and in the numerous functions fulfilled by the fingers. For such reasons the nails are protected and looked after. Their growth is stimulated most by the activity of the fingers themselves. In other words, the more the nails are used, the faster they grow. The surrounding tissues are fundamental to the health of the nails and, therefore, must be protected and not removed. As the nail lamina is composed of hard keratin it is therefore impermeable and can be covered with enamel and other substances which can at times be useful in increasing the nail's strength.
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1. Significant species difference exists between sheep and rat
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