i2 4 HEAD AND NECK 



hyoid to the mylo-hyoid line of the mandible. In front of the 

 anterior belly of the digastric, the two layers blend with the 

 single layer of deep fascia which covers the lower surfaces of 

 the mylo-hyoid muscles. Behind the posterior belly of the 

 digastric they unite with the connective tissue in which the 

 carotid vessels are embedded. 



When the details of the deep fascia have been examined, the 

 sterno-mastoid should be studied. 



M. Sternocleidomastoideus. The sterno-mastoid muscle 

 lies between the anterior and posterior triangles of the neck 

 (Fig. 43). It is attached, below, by two heads a sternal 

 and a clavicular. The sternal head is rounded, and chiefly 

 tendinous ; it springs from the upper part of the anterior 

 surface of the manubrium sterni. The clavicular head is 

 broad and fleshy, with only a few tendinous fibres intermixed ; 

 it arises from the medial third of the upper surface of the 

 clavicle. A narrow interval filled with fascia separates the 

 heads below, but at a higher level the sternal portion over- 

 laps the clavicular, and half-way up the neck the two heads 

 unite into a fleshy mass which ascends to the mastoid portion 

 of the temporal bone and occiput. There the muscle 

 expands somewhat. At its insertion it is thick and tendinous 

 where it is attached to the fore-part and lateral surface of the 

 mastoid process ; posteriorly it is thin and aponeurotic, and 

 is inserted into rather more than half of the corresponding 

 superior nuchal line of the occipital bone. In the dissection 

 of the back, the latter part of the muscle was detached from 

 the occiput. 



The dissectors should note that the insertion of the 

 sterno-mastoid into the skull is mainly posterior to the trans- 

 verse axis of rotation of the atlanto-occipital joint. There- 

 fore if one sterno-mastoid acts the head is drawn downwards 

 to that side and the face is turned to the opposite side and 

 tilted upwards. If both sterno-mastoids act simultaneously 

 the head is drawn backwards. The muscle is supplied by 

 the spinal part of the accessory nerve and by the second 

 cervical nerve. 



Dissection. Turn the anterior border of the sterno-mastoid 

 backwards and search for the arteries which supply it. At the 

 level of the angle of the mandible the sterno-mastoid branch of 

 the occipital artery will be found entering the deep surface of 

 the muscle. 



At the level of the cricoid cartilage the sterno-mastoid branch 

 of the superior thyrcoid artery enters the muscle, and a short 



