Merkle’s touch-cells are oval nucleated bodies in which the medullary nerve terminates, and are situated in the epidermis and upper corium. They are found in regions where tactile corpuscles are few, such as the abdominal surface. Their exact nature and functions are still a matter of dispute, although they have been studied in the lower animals as well as in men.
Both straited and smooth muscles are to be found in the skin, the former being less abundant than the latter. The straited or voluntary muscles of the skin are chiefly limited to the face and neck and arise from the subcutaneous tissue and the deep-seated muscles, and extend obliquely or vertically between the glands into the corium. To a certain extent expressions of the features can be traced to the action of these muscles in response to various physical and mental emotions. Their analogues in several of the lower animals are large and abundantly distributed.
Non-straited, smooth or involuntary muscles of the skin are very numerous and are found in the corium occupying horizontal and oblique positions in relation to the surface. The former run either in a straight or circular direction and are chiefly found in the scrotum, penis, areola and nipples of the breast, and eyelids. Contraction of these muscles forces the skin into folds and changes its external appearance. The oblique muscles are found in nearly all parts of the corium, either as minute fasciculi without attachment to the hair-follicles, or as more distinct muscular bundles with attachments to several adjacent hair- follicles below, and to the papillary layer above.
The latter group are known as the arrectores or erectores pilorum. These follicular muscles not only have fixed points of attachment to the papillary layer of the corium and fixed points of insertion into several hair-follicles at a level just above the apex of the hair-papillae, but by means of elastic fibers which surround and mingle with them throughout their length and at their ends form tendons, they are directly attached on every side to the elastic framework of the corium. Their direction is oblique. The direction of the hair being at a less oblique angle, a powerful contraction of these muscles pulls the hair into a more erect position. In some of the lower animals this effect may be seen in a marked degree. The more important effects which follow the ordinary contraction of the oblique muscles of the corium are the expulsion of sebum by compression of the sebaceous glands, a lessened circulation of blood in the papillary layer, and diminished perspiration from the general tension of the upper part of the corium. The compression exerted upon the skin in this way sometimes produces an apparent roughening of the surface (cutis anserina or goose flesh). A general effect is to prevent loss of bodily temperature. Therefore, one of the functions of the oblique muscles of the skin is the regulation of temperature. External cold stimulates their contraction and external heat promotes their expansion.
The shade of color of the skin is due partially to the degree of vascularity and distention of the blood-vessels in the corium. In other words it depends upon the quantity of blood circulating in the skin. The other element is the amount of staining in the cells of the lower strata of the rete, most pronounced in the nuclei and due to the deposit of fine granules of pigment (melanin) in the cell cavity. The amount of coloring matter in the layers of the epidermis is influenced somewhat by exposure to the sun, habits, climate, racial and other differences.