•292 MUSCLES OF THE HEAD AND NECK. 



in the posterior triangle, invegting- the posterior belly of the omo-hyoid 

 mnscle, and binding it down to the clavicle and first rib, where tiiis 

 structure is connected with the costo-coracoid membrane. Still deeper 

 in the anterior triangle the fascia passes behind the depressor muscles 

 of the larynx, investing the thyroid body, and extending thence on the 

 trachea and large vessels at the root of the neck down to the fibrous 

 layer of the pericardium. 



'Continuous with the deep processes of the cervical fascia is the common 

 shcaih of fhe large cervical blood vessels, an envelope of fascia enclosing 

 the carotid artery and jugular vein with the pneumogastric nerve. A 

 thin fibrous septum intervenes between the artery and vein, thus com- 

 pleting a separate sheath for each. 



The layer of fascia descending on the prevertebral muscles, and inter- 

 vening between them and the pharynx and oesophagus, is called the 

 j^rererlehral fascia. 



Muscles. — The sterno-cleido-mastoid or sterno-mastoid muscle is 

 attached inferiorly in two parts to the anterior surface of the sternum 

 and the inner third of the clavicle. The sternal attachment is thick and 

 rounded, tendinous in front and fleshy behind. The clavicular portion, 

 separated from the sternal by a narrow interval, is flat, and is composed 

 of fleshy and tendinous fibres. Those two portions become blended 

 together about the middle of the neck into a thick and prominent muscle, 

 which, extending upwards and backwards, is attached superiorly to the 

 anterior border and external surface of the mastoid process, and from 

 thence backwards into a rough ridge of the temporal bone, and by a 

 thin aponeurosis into the outer part of the superior curved line of the 

 occipital bone. The sterno-mastoid muscle divides the quadrilateral 

 space on each side of the neck into two great triangles. 



ItcJatiom. — This muscle is covered for more than the middle three-fifths of its 

 extent by the platysma. It is also crossed by the external jugular vein, and by 

 the ascending superficial branches of the cervical plexus of nerves. It rests 

 on part of the sterno-hyoid and sterno-thyi'oid muscles, and crosses the omo-hyoid 

 muscle ; in the middle part of the neck it covers the cei-vical plexus and the 

 great cervical vessels, and in the upper part, the digastiic and stylo-hyoid muscles : 

 it is pierced by the spmal accessory nerve. 



Vtirhtirs. — The stemo-cleido-mastoid is occasionally described as two muscles, 

 under the names stemo-mastoid and cleido-mastoid. Nonnally the fibres of the 

 clavicular part run upwards for some distance behind and inside those of the 

 sternal part before finally blending -ndth them, and sometimes they are separated 

 at the root of the neck by a considerable interval which is occupied by connective 

 tissue. Tire muscle varies much in breadth at the lower end, the variation 

 being due altogether to the clavicular part, which is sometimes as naiTow as the 

 sternal tendon, wliile in other instances it extends for three inches along the 

 clavicle. This part of the muscle may likewise, when broader than usual, he 

 divided into several slips separated by intervals near the clavicle. A band of 

 muscvUar fibres has, in a few instances, been found reaching from the trapezius to 

 this muscle o-^^er the subclavian artery ; and the margins of the two muscles have 

 been ol)served in contact. In animals without a clavicle the cleido-mastoid 

 muscle is continued into the clavicular part of the great pectoral muscle, thus 

 forming a iiuintoido-kiimcral muscle. 



The four following muscles may be classed together as a group of 

 Depressors of the Hyoid Bone and Larynx. 



The sterno-hyoid muscle, a flat band of longitudinal fibres, arises 

 variously, from the sternum and the posterior sterno-clavicular ligament, 

 from the clavicle and that ligament, or from the clavicle only, and 



