426 THE MUSCLES 



peculiar in the fact that many of them decussate with one another on the Avay to 

 their insertion in the skin; and they are all supplied by the seventh pair of 

 cranial nerves. 



SUPERFICIAL MUSCLES OF NECK AND SCALP 



1. PLATY8MA MYOIDES 



The platysma myoides — named from its flat expanse {platysma, a plate) and 

 its similarity to muscle {ini/oides, like muscle, for the older anatomists considered 

 it to be only a membrane) — is a quadrilateral sheet with a somewhat toothed 

 posteri(^r 1>order. 



Origin. — (1) The deep fascia of the upper part of the front of the chest and 

 the shoulder, and the deep cervical fascia covering the sterno-mastoid. 



Insertion. — (1) The outer surface of the lower border of the body of the 

 mandible, bt)th on the same and the opposite side; (2) the deep surface of the skin 

 from the corner of the mouth to the anterior border of the masseter muscle. 



Structure. — Arising by slender bundles from the fascia covering the pectoralis 

 major and deltoid, in a line from the second costal cartilage to the tip of the 

 acromial process, the pale muscular fibres at first converge slightly until they have 

 crossed the clavicle. They then run parallel to one another upwards and forwards, 

 receiving at the posterior border of the muscle small teeth from the deep fascia 

 forming the sheath of the sterno-mastoid. The anterior fibres cross the middle line 

 half-way between the hyoid bone and the symphysis, and are inserted into the 

 lower border of the body of the mandil:)le upon the opposite side of the neck. The 

 greater part of the rest are inserted into the outer surface of the lower border of 

 the mandible on the same side, but some are continued upwards and join Avith the 

 depressor labii inferioris. Still fvirther back, a few fibres pass upwards over the 

 lower part of the masseter, and are inserted into the deep surface of the skin just 

 outside the angle of the mouth. A strong bundle of these fibres which runs nearly 

 transversely forwards to the corner of the mouth will be described later as the 

 risorius muscle. In its whole extent, the muscle is closely connected with the deep 

 surface of the skin. 



Nerve-supply. — The seventh cranial nerve, by means of the inframandil)ular 

 branch of the cervico-facial division, which supplies it from beneath at a point a 

 little below the angle of the mandible. 



Action. — (1) To draw downwards and outwards the lower lip, by means of the 

 fibres which are continued upwards into the depressor labii inferioris. In trans- 

 verse wounds below the mandible, the division of this muscle often impairs this 

 movement. Duchenne has shown how important a part this depression of the 

 lower lip discbarges in the expression of horror or extreme surprise. (2) To draw 

 outwards the soft parts of the side of the neck, so as to diminish pressure upon the 

 veins during strong inspiratory efforts. As the muscle is arched in its ])assage 

 from the clavicle to the mandible, with the concavity directed outwards, it is plain 

 that its contraction will flatten the arch and tend to lift the skin and fascia' off the 

 vessels, in exactly the opposite manner to that by which pressure is exerted when 

 the concavity is upon the deep surface of a muscle. (3) It may assist feebly in 

 depression of the mandible; and, if that be fixed, in flexion of the head and rotation 

 towards the same side. This muscle covers all the front of the neck with the 

 exception of a narrow triangular interval, the base of which is formed by the upjun* 

 Vjorder of the sternum and the adjacent sterno-clavicular joints. 



Relations. — It lies inmiediately bcneatli the skin and superficial fascia, and in 

 a lean neck the bundles are often distinctly visible through tlie skin, esi^ecially 

 when an effort is made to depress the lower li]) while the teeth are clenched. 

 A thin band derived from the anterior part of tlie dei)ressor anguli oris usually 

 crosses it superficially beneath the chin (page 48<S). Upon its deep surface lie 

 the deep cervical fascia, the external jugular vein, the glanduUc concatenatfe of the 

 neck, the parotid and submaxillary glands, and the superficial branches of the 



