288 



THE MUSCLES OF THE HEAD AND NECK. 



median portion of the lip, a few on the same side, but the greater number 

 after crossing the middle line and decussating with their fellows : they do not 

 reach the free border of the lip, but leave the marginal bundle of buccinator 

 fibres uncovered. Besides these fibres, there are others which are attached to 

 the cartilage and bone, viz., in the upper lip two slips, and in the lower lip 

 one slip for each half. In the upper lip, the outer slip (m. incisivus sup.}, thin 

 and weak, springs from the incisor fossa of the superior maxilla ; while the other 

 (m. naso-labialis), thick and pointed, is fixed to the hinder part of the septum 

 narium. In the lower lip the reinforcing fasciculus (m. incisivus inf.) arises from 

 the incisor fossa of the inferior maxilla outside the levator menti. In both lips the 



CRBICULARIS 



Fig. 271. SAGITTAL SECTION OP UPPER LIP OF 



INFANT, A LITTLE TO ONE SIDE OF THE 

 MEDIAN PLANE. (Aeby.) 



slips are directed outwards towards the 

 corner of the mouth, and there blend 

 with the rest of the muscle. True orbi- 

 cular fibres, passing from lip to lip, 

 probably do not exist. 



In front of, and to some extent interlac- 

 ing with, the transverse fibres above de- 

 scribed are the vertical fibres of the elevator 

 and depressor muscles of the respective lips, 

 passing to their insertion into the skin. 

 There is also in the marginal portion of 

 each lip a special set of fibres (M. Idbii 

 propr'ms Krause), more developed in the 

 infant, passing obliquely from the skin to 

 the mucous membrane between the trans- 

 verse and vertical fibres. (W. Henke, 

 Zeitsch. f. Anat. u. Entw., i, 107 ; C. Aeby, 

 Arch. f. mikrosk. Anat., xvi, 651.) 



Relations. The skin of the lips is 

 closely connected to the inner part of the 

 orbicularis oris muscle, while over the outer 

 part fatty tissue is interposed between them. 

 The deep surface is in contact with the 

 mucous membrane and the labial glands, 

 as well as with the coronary arterial arch 

 in each lip. 



Nerves. All the superficial muscles of the scalp and face previously described receive 

 their motor fibres from the facial or seventh cranial nerve. It is probable, however, that the 

 fibres which supply the f rontalis and orbicularis palpebrarum muscles have their central origin 

 from the oculo-motor nucleus, and not from that of the facial nerve (Mendel, Neurolog. 

 Centralblatt, 1887, p. 537). 



Actions of the muscles of the lips. The orbicularis oris draws the lips together in both 

 the vertical and transverse directions. Its deeper part, acting with the buccinators, closes the 

 lips and presses them against the teeth. Its outer and superficial portion, continued from the 

 muscles of the angle of the mouth, shortens the aperture, brings the lips together, and causes 

 them to project forwards. 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 Up and nose 

 and the depressor al(e nasi act on the upper lip and the wing of the nose together the one raising, 

 the other depressing them. When the cheeks are distended with air. and the aperture of the 

 lips narrowed, it is by the action of the buccinator that the forced expulsion of the air is 

 regulated. The buccinator also flattens the cheek and keeps the food between the teeth during 

 mastication. The levator menti draws upwards the integument of the chin and protrudes the 

 lower lip. 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 



ABIAL GLANDS 



ARY ARTERY 



GINAL BUNDLE 



