MUSCLES OF THE HEAD. 219 



Situation Form. Situated on the sides of the face, partly concealed by 

 the masseter muscle, and applied to the mucous membrane of the cheeks, the 

 alveolo-labialis is a flat, thin muscle, elongated in the direction of the head, 

 and formed of two superposed planes. 



Extent Structure Attachments. The deep plane, the longest and least 

 wide, is narrower at its extremities than its middle, and is formed of strongly 

 aponeurotic muscular fasciculi, which are attached, posteriorly : 1, To the 

 alveolar tuberosity ; 2, To the external surface of the superior maxillary 

 bone, above the last three molar teeth; 3, To the anterior border of the 

 inferior maxillary bone, behind the sixth molar, in common with the maxillo- 

 labialis. On reaching the commissure of the lips, this muscular layer 

 appears to be continued by small tendons with the fibres of the orbicularis. 



The superficial plane only begins about the middle of the deep one, whose 

 anterior half it entirely covers. Its fibres, less tendinous than those of the 

 latter, extend from a median raphe which also divides the deep layer in its 

 length, and are directed, some forwards, some backwards, to terminate in the 

 following manner : the first are inserted into the external face of the super- 

 maxillary bone, above the first molar tooth and the superior interdental 

 space ; the second are attached to the inferior interdental space alone. 



Relations. Externally, with the masseter, zygomatico-labialis, cuticularis, 

 great supermaxillo-nasalis, supernaso-labialis, the parotid duct, which crosses 

 it to enter the mouth, and the facial artery and veins ; internally, with the 

 buccal mucous membrane. The deep plane is accompanied and covered at its 

 anterior border by the upper molar glands ; its posterior border is margined 

 by "the inferior molar teeth, which it partially covers. The superficial layer 

 is distinctly separated from the deep one in its anterior part, which is attached 

 to the superior maxillary bone. Behind, these two planes adhere more 

 intimately to one another, though they are found completely isolated by an 

 interstice in which one or two large veins pass. 



Actions. The function of the alveolo-labialis is particularly related to 

 mastication : this muscle, in fact, pushes between the molar teeth the portions 

 of food which fall outside the alveolar arches ; but it cannot aid in bringing 

 the two jaws together, as M. Lecoq has correctly observed. 



3. Zygomatico-labialis. (Fig. 110, 21.) 



Synonyms. Portion of the cuticularis of Bourgelat. The zygomaticus major of Man. 

 (Zygomaticus Percivall.) 



A very small, pale, and thin ribbon-like muscle, arising from the surface 

 of the masseter, near the maxillary spine, by an aponeurosis which is con- 

 founded with the cuticularis ; it terminates on the surface of the alveolo- 

 labialis, at a short distance from the commissure of the lips. Covered by 

 the skin, it covers the alveolo-labialis muscle, and some of the superior molar 

 glands, vessels, and nerves. 



This muscle pulls backwards the commissure of the lips when it is in a 

 state of contraction. 



In Solipeds there is also sometimes found a muscle resembling the 

 zygomaticus minor of Man. It is a very small fasciculus situated under the 

 preceding muscle, near its superior extremity. It appears that this fasciculus 

 is continued, above, by the fibres of the lachrymo-labialis, and is lost, below, 

 on the alveolo -labial surface, a little beneath the carotid canal. 



