178 MUSCLES OF THE HEAD AND NECK. 



The depressor labii inferioris, or quadratus menti, arises from the lower 

 jaw by a line of attachment extending from near the symphysis to a little 

 beyond the mental foramen ; thence it ascends with an inward inclination, 

 unites with its fellow, and blending with the orbicularis oris is inserted into 

 the lower lip. Its fibres are intermixed with much adipose matter. 



It covers the nerve and artery issuing from the mental foramen. 



The levator labii inferioris or levator menti arises by a narrow head from 

 the incisor pit of the lower jaw, and, expanding in a nearly vertical plane, is 

 directed forwards, downwards, and slightly inwards, to the integument of 

 the chin between the depressors of the lower lip. 



ACTIONS. The orbicularis oris acting alone draws the lips together in both the 

 vertical and transverse directions. Acting in conjunction with the buccinators it 

 closes the lips, while at the same time they are elongated transversely. Its facial 

 portion acting alone projects the lips. The labial portion, when acting in concert 

 with the converging muscles, tightens the lips, one or both, against the teeth. The 

 convergent muscles each draw their oral points of insertion in a direction corresponding 

 to that of their muscular fibres. The common elevator of the lip and nose and the 

 upper part of the orbicularis oris act on both the upper lip and the ala of the nose 

 together the one elevating, the other depressing them. When the cheeks are dis- 

 tended with air in the mouth, it is by the action of the buccinator that the rush of 

 the air through the narrowed oral aperture is regulated, as in whistling, or performance 

 on a wind instrument ; hence the name " trumpeter's muscle " sometimes given to 

 the buccinator. The levator menti not only draws upwards the integument of the 

 chin, but it also protrudes the lower lip, as in pouting. The muscles attached to the 

 angles of the mouth are, along with others of the face, intimately connected with 

 the expression of the passions : those which pass downwards not only raise the upper 

 lip, but also push upwards the cheek, and thus elevate the margin of the lower eyelid, 

 as in laughter : and those which descend from the angle of the mouth depress that 

 part, as in weeping. (On the action of the facial muscles may be consulted, more 

 especially, Sir Charles Bell, " Anatomy and Philosophy of Expression.") 



THE PLATYSMA MYOIDES is a pale coloured thin sheet of muscular fibres, 

 superficial to the deep cervical fascia, and extending over the front and 

 sides of the neck and lower portions of the side part of the face. Its 

 fibres, which are pale and thin in their whole extent, rise by thin bands 

 from the clavicle and acromion, and from the fascia covering the upper part 

 of the deltoid, pectoral, and trapezius muscles ; thence they proceed up- 

 wards and inwards over the clavicle and the side of the neck, gradually 

 narrowing and approaching the muscle of the opposite side. At the lower 

 jaw the greater number of the fibres are inserted into the side of that bone 

 from the symphysis to the attachment of the masseter ; the inner fibres 

 mingle with those of the opposite platysma in front of the symphysis, and 

 even cross from the one side to the other below the chin, for about an inch, 

 those of the right side overlapping those of the left ; and the posterior 

 fibres are prolonged upon the side of the face as far as the angle of the 

 mouth, where they become blended with the muscles in that situation, or in 

 some instances they reach the fascia over the parotid gland, and the cheek- 

 bone. The uppermost fibres pass into those receiving the name of risorius. 



ACTION. The platysma, being much less developed in man than in animals, has a 

 comparatively limited action in the human subject; it assists in drawing the angle of 

 the jaw downwards and outwards, and protects parts more deeply situated in the 



