Opposite, top: Facial expressions are achieved by movements of the soft facial tissue.
The musculature of the face is basically quite simple, although there are several layers of muscles which allow very subtle movements of the facial tissue, and thereby facilitate facial expression. These muscles are attached at one end to bone and at the other to the skin and its concomitant subcutaneous flesh. Pursing and pouting of the lips, opening and closing of the eyelids, etc., are governed by the sphincter-type muscles around the mouth and eyes.
A great many learned works concerning human facial expression have been published during the past 300 years, the most notable being Expression of Emotions in Man and Animals (1873) by Charles Darwin (1809-1882), in which he concludes that the manner in which human beings show their emotions is further proof of his theory of evolution. Of a little more interest to the artist is the work of the French anatomist Guillaume Duchenne (1806-1875), who attempted to isolate the expressive function of each of the facial muscles; however, most of the time several muscle groups work in combination to achieve each of the wide range of subtle facial expressions.
Constant observation, coupled with an understanding of bone structure and surface soft tissue, is the key to understanding facial expression.
Over the years, the soft, resilient, firm skin of the child becomes replaced by the crinkled, limp and blemished skin of old age. The subcutaneous fat layer immediately beneath the skin becomes thinner and the facial flesh of the cheeks and chin sags over the bottom edge of the mandible, forming the distinctive drooping jowls and double-chin of the elderly. The fat-cushions behave, in later life, like loose, water-filled bags, and become more distinctly separated from one another and from the overall flesh layer under the skin.
The nasolabial furrow deepens as a consequence, and facial lines and wrinkles develop in specific and characteristic locations. These are illustrated here; they include 'crow's feet' at the outer corners of the eyes, horizontal lines on the forehead, vertical corrugations on the bridge of the nose between the eyes, and vertical creases along the upper lip. All the fat-cushions listed earlier become increasingly well defined as separate topographical features by creases at their inner and lower edges.
The skin under the chin and on the front of the throat, with its associated flesh, sags. Two factors cause the tightening of this area: the natural wearing down or loss of the teeth brings the jaws closer together, so that the chin juts forward more, and the increased forward curvature of the spine between the shoulderblades throws the head forward and down, so that the neck vertebrae have to adopt a greater compensating backward curve. These two factors cause a tightening of the skin under the chin.
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