THE FACIAL MUSCLES. 



481 



thence over the head and neck. At first they are confined entirely to the neck region, 

 but even in the lower mammals the extension upon the head has begun, and in the 

 higher members of this group two portions can be distinguished in the muscle-sheet. 

 The more superficial of these is situated in the lateral and posterior portions of the 

 neck, and extends thence upon the sides of the face and over the vertex of the skull 

 to the orbital and nasal regions of the face. The deeper one lies more anteriorly in 

 the neck, and extends upward over the jaw to the region around the mouth. 



In the higher forms a differentiation of both layers to form a number of more 

 or less separate muscles takes place and reaches its highest development in man, 

 whose mobility of facial expression is due to the existence of a considerable number 

 of platysma muscles. These muscles have arisen from the common sheets by the 

 partial conversion of these into connective tissue, by the secondary attachment of 

 portions of the sheets to the skeleton, by xarious modifications of the primary direc- 

 tion of the fibres, and by the obliteration of certain portions of the sheet found in the 



• 



Fig. 498. 



Depressor anguli oris 



Mandible — 



Raphe of mylo-hyoid 



Sterno-hyoid ^ .^r 



— -_ ^Depressor labii inferioris 



— Levator menti 



ki 



r X 



Platysma- 



i ^Mylo-hyoid 



_Hyoid bone 

 —Thyroid cartilage 



Sterno-mastoid 



Sterno-thyroid 



Inner end of clavicle 



Superficfal muscles of neck. 



lower animals, the cervical portion of the deep layer, for instance, being normally 

 lacking in man, the layer being represented only by the muscles of the lips. 



The platysma musculature is characterized for the most part by the pale color of 

 its fibres, by their aggregation to form thin bands or sheets usually more or less inter- 

 mingled with connective-tissue strands, so that their margins are, as a rule, ill-defined, 

 and by their attachment in frequent cases to the integument. These peculiarities, 

 together with a considerable amount of variation which occurs in the differentiation of 

 the various muscles, have brought about not a little difference in the number of muscles 

 recognized in the group by various authorities, some recognizing as distinct muscles 

 what others regard as merely more or less aberrant or unusually developed slips. 



(a) THE MUSCLES OF THE SUPERFICIAL LAYER. 



I. Platysma (Figs. 498, 499). 



Attachments. — The platysma takes its orio-m from the skin and subcutaneous 

 tissue over the pectoralis major and deltoid muscles on a line extending from the car- 

 tilage of the second rib to the tip of the acromion process. • Its fibres are directed 



31 



