Embryologic, Histologic, and Anatomic Aspects
In the skin, smooth muscle occurs in two different unrelated settings. One is in fascicles that form muscles of hair erection and the other is in the wall of arterioles and venules. The fascicles that make up the arrector pili muscle originate from bulges of the isthmus and the upper part of the stem of a follicle and seem to insert at the base of epidermal rete ridges. That muscle serves to erect the hair and, in the process, causes a tiny papule around the hair shaft to come into being, that being known by laypersons as a “goose bump.”
Erector smooth muscle unaffiliated with follicles is present in abundance in the breast, especially in the vicinity of the nipple, in the labium majus, and in the scrotum, imparting to those anatomic sites distinctiveness histologically.
No muscles of hair erection are found in the subcutaneous fat, but muscle is present in abundance in the wall of arteries and veins housed in fibrous septa there. The wall of vessels is made up principally of (1) the intima, which consists of a thin lining of endothelial cells and of a thin layer of connective tissue (which, in arteries, also contains an internal elastic membrane), (2) the media, which is made up of smooth muscle cells and elastic tissue, and (3) the adventitia, which is composed entirely of connective tissue. Arteries and veins differ in the thickness of the muscular layer, the wall of arteries usually being considerably thicker than that of veins.
Skeletal muscle is not a constituent of the skin, but is encountered in the superficial part of the subcutis on certain anatomic sites, especially the head and neck and, particularly, the eyelids and the upper lip. The muscles of the face, in general, have as their origin either the fascia or the bones of the face and as their insertion the dermis. As but one example, a muscle seen often in sections of tissue cut from biopsy specimens taken from the eyelid is the musculus orbicularis oculi, which arises from the nasal part of the frontal bone and the frontal part of the maxilla. The muscle fibers form a broad, but thin, layer that resides in the eyelids, surrounds the circumference of the orbit, and extends over the temple and down the cheek. The muscle serves as a sphincter for the eyelids and is responsible for the act of blinking. When the entire muscle is brought into motion, the skin of the forehead, temple, and cheek is drawn toward the medial angle of the orbit. The skin of the lateral angle of the eyelids is thrown, thereby, into folds, which, when they become permanent, as they tend to do in the elderly, are known as “crow”s feet.”