274 



COMPARATIVE ANATOMY 



ANTERIOR 

 AURICULAR' 



nerve, and used chiefly for the expression of the emotions. The facial 

 muscles arise chiefly, if not exclusively, from the mesoderm of the hyoid 

 arch, from which they spread to the face and scalp. Although most 

 visceral muscles are involuntary, those of the face are under the control 

 of the will. 



Among the best developed of the facial muscles are those connected 

 with the lips. The platysma is a broad sheet extending from the corners 



of the mouth along the sides of the 

 neck. When it contracts it de- 

 presses the corners of the mouth as 

 in grief. The orbicularis oris 

 encircles and closes the mouth. The 

 risorius, connected with the corners 

 of the mouth, is not a laughing 

 muscle as its name suggests, but is 

 used in drawing back the corners as 

 in pain or grief. The buccinator, 

 which also radiates from the corners 

 of the mouth, is used in compressing 

 the cheeks to keep food between 

 the teeth. The caninus raises the 

 corners of the mouth as in sneering. 

 The zygomaticus runs from the 

 corners of the mouth to the zygo- 

 matic bone and is the true smihng 

 muscle. The triangularis depresses 

 the corners of the mouth as in grief. 

 A few weak slips of muscle surround the nasal orifices, among which the 

 nasalis functions both as constrictor and dilator of the nares. 



One of the facial muscles, the orbicularis oculi, closes the eyeUds. 

 But the muscle' which raises the upper lid, the levator palpebrae superioris, 

 is innervated by the oculomotor nerve and is therefore not regarded as 

 one of the facial muscles. The corrugator muscle, however, which serves 

 to wrinkle the forehead in frowning, is a facial muscle. 



Of the muscles of the scalp, the frontalis draws the scalp forward and 

 wrinkles the forehead, the occipitalis draws the scalp back. A number 

 of auricularis muscles retract or raise the ears. The six eye muscles, — 

 the superior, inferior, internal, and external rectus, and the superior and 

 inferior oblique, do not belong to the integumentary group. The function 

 of the obhque muscles is thought to be to prevent rotation of the eyeball 

 when the rectus muscles contract. 



A number of visceral muscles associated with the jaw and innervated 

 by the trigeminal nerve are used in mastication. These include the 



ANTITRAGICUS/ AURICULAR 



Fig. 228. — The auricular muscles in 

 man. The presence of such useless 

 muscles in man and their similarity to 

 those of other mammals has an evolu- 

 tionary significance and interest. 



