70 A MANUAL OF ANA TO MY. 



3. Continue the incision from the lobe of the ear along the line of its pos- 

 terior attachment to the head . 



Reflect the integument from behind the ear, front, and side of the neck, so 

 as to expose the anterior border of the trapezius muscle. 



Platysma Myoides. Fig. 14. 



Origin. From the deep fascia covering the upper part 

 of the pectoralis major, deltoid, and trapezius muscles, and 

 slightly from the front of the upper surface of the clavicle. 



Insertion. Into the anterior three-fourths of the lower 

 border of the inferior maxilla (the fibres of the muscle cross- 

 ing the median line at the point of the chin), and into the 

 angle of the mouth. 



Nerve Supply. The superficial cervical, from the second 

 and third cervical nerves, and the inframaxillary branch of 

 the cervico-facial division of the seventh. The latter is the 

 main supply. 



Action. To depress the lower jaw. This being fixed, 

 they will flex the head and retract the angle of the mouth. 

 If one acts it will have the same action on one side of the 

 mouth, depress the jaw ; if this is resisted, the head will be 

 flexed laterally and the chin rotated to the same side. 



The upper part of the muscle being the fixed part, it 

 raises the skin over its origin, and in the neck tends to draw 

 it away from the deeper parts. 



DISSECTION. 



After completing the platysma, begin at its origin (below) and carefully 

 separate it from the deeper structures beneath. The nerve supply to the muscle 

 will be found, and, after being recognized, may be divided and the muscle 

 rolled up on to the lower part of the face. 



The first layer of the cervical fascia is now exposed ; the other layers will 

 follow in order and should be identified as far as possible. In order to do 

 this, consult the technical description of the fascia as found on page 136. 

 Its description here now would not be so easily understood as after the 

 dissection of the neck has been completed. 



