STERXO-CLEIDO-MASTOID 445 



it rests in the middle line \\\^(n^. the ligaments covering the front of the bodies of 

 the cervical vertebrse. Laterally, it covers in the prevertebral muscles, and is 

 attached to the deep surface of the superjicial layer of the cervical fascia between the 

 sterno-mastoid and trapezius muscles. • 



In the c()m])artment formed l)etween the deep layer of the cervical fascia as it 

 invests the depressors of the hyoid ])one, and the preverteljral fascia, processes are 

 given off which form the sheath of the great vessels of the neck, and invest the 

 thyroid gland, the trachea, and pharynx. 



"^ The muscles of the front of the neck may be divided into three groups: the 

 first group consisting of one muscle which ascends from the sternum and clavicle 

 to the head, the sterno-cleido-mastoideus; the second, of those which ascend from 

 the sternum, clavicle, and shoulder-blade to the hyoid bone and thyroid cartilage; 

 the third, of those which are attached to the hyoid bone below, and the skull and 

 lower jaw above. 



First Group 



STERXO-CLEIDO-MASTOID 



The sterno-cleido-mastoid, or sterno-mastoid muscle — named from its 

 attachments {y.ht^=:a key, being the equivalent of clavicle) — is a strong ribbon- 

 shaped band, l)ifurcated below, and somewhat constricted in its middle third. 



Origin. — Sternal head : the front of the manubrium (or presternum) Ix'tAveen 

 the notches, th(^ middle line, and for the clavicle and first rilx Clavicular head : 

 The upper part of the anterior surface of the inner third of the clavicle. 



Insertion. — (1) Along the anterior border and the outer surface of the mastoid 

 process of the temporal bone; (2) the outer half of the superior nuchal line of the 

 occipital bone. 



Structure. — The sternal head is a rounded but flat tendon; the clavicular is 

 partly fleshy and partly tendinous. After a course of about an inch (2-5 cm.), the 

 sternal head expands into a flat muscle, which conceals the greater part of the 

 clavicular portion, and, passing upwards, outwards, and backwards, is spread over 

 the whole line of the upper attachment. Frequently it is so separate from the 

 clavicular head, that they might very fairly l)e considered to form two muscles. 

 The clavicular head soon becomes entirely fleshy and ascends more directly. At 

 first it is separated by a small interval from the sternal head, corresponding to a 

 part of the sterno-cla\dcular joint, from which it sometimes receives a few fibres of 

 origin; when they have joined, it passes beneath the sternal head to its insertion, 

 which is chiefly the lower part of the outer surface of the mastoid process. The 

 whole insertion in front is composed of short tendinous fil)res, and behind of 

 a thin aponeurosis. As the w'hole muscle has a ver}- wide range of action, 

 nearly the whole length of its fibres is fleshy. The sternal head is a little 

 longer than the other, but has little if any more range of movement. Hence it 

 is tendinous. 



Nerve-supply. — (1) The spinal accessory nerve, which, while traversing the 

 deep surface of the muscle at the junction of its upper and middle thirds, sends 

 filaments to it; (2) the cervical plexus through the anterior primary branches 

 of the second and third cervical nerves, which enter the upper part of its deep 

 surface. 



Action. — (1) To flex laterally the head and neck, so as to draw the side of the 

 head towards the shoulder. (2) To rotate the face towards the oi)posite side. Of 

 the two parts of the muscle, the cleido-mastoid portion is more concerned in lateral 

 flexion, the sterno-mastoid in rotation. The combination of all these movements 

 may be very well seen in a case of wry-neck, which results from the permanent 

 contraction of this muscle. (3) "When both muscles act, to flex the head and neck 

 upon the thorax, at the same time raising slightly the chin, which is therefore 

 carried horizontally forwards. (4) To raise the sternum and inner end of the 

 clavicle. This action may sometimes be seen in patients with paralysis of all the 

 parts beneath the cervical region, when the only nerves available for respiratory 



