298 THE MUSCLES AND FASCIA OF THE HEAD AND NECK. 



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 carotid sheath, 

 an envelope of fascia enclosing the carotid artery and jugular vein with the pneumo- 

 gastric nerve. A thin fibrous septum intervenes between the artery and vein, thus 

 completing a separate sheath for each. 



Deepest of all is the prevertelral fascia, a layer descending on the prevertebral 

 muscles, and forming the hinder wall of what may be termed the visceral compart- 

 ment of the neck, which lodges the pharynx and oesophagus, the larynx and trachea, 

 with the thyroid body. Covering the back of the pharynx there is a proper fascial 

 layer (p. 306), adhering closely to the constrictor muscles, but connected behind to 

 the prevertebral fascia only by very loose areolar tissue, the meshes of which are 

 readily distended by fluid, thus giving rise to the so-called retro-pharyngeal space. 

 This space is bounded laterally by the union of the pharyngeal and prevertebral 

 fasciae, both of which join the inner side of the carotid sheath, while inferiorly 

 the loose tissue is continued without interruption along the oesophagus into the 

 thorax, so that the artificial cavity may be extended downwards into the posterior 

 mediastinum. Lastly, from the back of the carotid sheath a layer, which may be 

 looked upon as the prolongation of the prevertebral fascia, extends outwards and 

 downwards over the scaleni muscles, the brachial plexus of nerves, and the sub- 

 clavian vessels, which it accompanies into the upper part of the axilla, thus giving 

 rise to the axillary sheath. Between this layer and the more superficial layer roof- 

 ing over the lower part of the posterior triangle is an interval in which are con- 

 tained the posterior belly of the omo-hyoid with its fascia of attachment, the 

 transverse cervical and suprascapular vessels, the inferior deep cervical lymphatic 

 glands, and fat. 



MUSCLES. Immediately beneath the fascia is a large oblique muscle, extending 

 the whole length of the neck, and named from its attachments the sterno-cleido- 

 mastoid. At a deeper level than this are two sets of muscles situated respectively 

 above and below the hyoid bone. The muscles of the upper set are known as the 

 suprahyoid muscles or elevators of the hyoid bone ; those of the lower set are the 

 infrahyoid muscles or depressors of the hyoid bone. 



The sterno-cleido-mastoid or sterno-mastoid muscle is attached inferiorly 

 in two parts to the anterior surface of the manubrium and to the inner third of 

 the clavicle on its upper aspect. The sternal head is thick and rounded, tendinous 

 in front and fleshy behind. The clavicular portion, separated from the sternal by 

 a variable interval, is flat, and is composed of fleshy and tendinous fibres. The 

 two portions meet and form a thick prominent muscle, which, extending upwards, 

 outwards and backwards, is inserted superiorly by short tendinous fibres into the 

 anterior border and external surface of the mastoid process, as well as into a rough 

 ridge continued backwards therefrom, and by a thin aponeurosis into the outer 

 half or more of the superior curved line of the occipital bone. The sterno-mastoid 

 muscle divides the quadrilateral space on the side of the neck into two great triangles. 



Relations. This muscle is covered for more than the middle three-fifths of its extent by 

 the platyema. It is also crossed by the external jugular vein, and by the great auricular and 

 superficial cervical nerves. It rests below on part of the stemo-hyoid and sterno- thyroid 

 muscles, and crosses the omo-hyoid ; in the middle part of the neck it covers the cervical 

 plexus, the great cervical vessels, and parts of the levator anguli scapulas and scaleni muscles, 

 and in the upper part, the splenius and digastric muscles. It is pierced by the spinal acces- 

 sory nerve. 



Varieties, &c. This muscle consists of two chief parts, sterno-mastoid and cleido-mastoid, 

 which are sometimes described as separate muscles. The cleido-mastoid, the smaller of the 

 two, runs upwards nearly vertically under the sterno-mastoid, and is inserted separately by a 

 rounded tendon into the tip of the mastoid process ; this part is pierced by the spinal accessory 



