THE HEAD AND NECK 1091 



the superior curved line of the occipital bone, where the stemo- 

 cleido-mastoid and trapezius may meet, but the apex is usually 

 truncated. The roof is formed by the skin, superficial and deep 

 fasciae, and, for a short distance inferiorh^ by the platysma myoides. 

 The small occipital nerve lies in the upper part of the roof, and the 

 descending superficial branches of the cervical plexus and the 

 external jugular vein lie in the lower part of the roof. The floor 

 is formed by the following muscles, in order from above down- 

 wards : (i) a small angle of the complexus, provided the trapezius 

 is not well developed at the occiput ; (2) the splenius capitis ; 

 {3) the levator anguli scapulae ; (4) the scalenus medius and scalenus 

 posticus ; (5) the posterior belly of the omo-hyoid ; and (6) the 

 scalenus anticus. 



The posterior triangle is subdivided by the posterior belly of the 

 omo-hyoid into a large upper portion, called the occipital triangle, 

 and a small lower portion, called the subclavian or supraclavicular 

 triangle. 



Occipital Triangle — Boundaries — Anterior. — ^The posterior border 

 of the sterno-cleido-mastoid. Posterior. —The anterior border of 

 the trapezius. Inferior (base). — ^The posterior belly of the omo- 

 hyoid. The muscles in its floor are (i) a small angle of the com- 

 plexus (inconstant), (2) splenius capitis, (3) levator anguli scapulae, 

 and (4) scaleni medius et posticus. The contents are the super- 

 ficial branches of the cervical plexus, the spinal accessory nerve, 

 the branches of the cervical plexus to the levator anguli scapulae 

 and trapezius, a small part of the occipital artery close to the apex, 

 and the glandulae concatenatae. 



Cervical Plexus. — ^The cervical plexus is situated under cover of 

 the upper part of the sterno-cleido-mastoid muscle, and lies 

 immediately in front of the slips of origin of the scalenus 

 medius. It is formed by the anterior primary divisions of the 

 first three cervical nerves and the greater part of that of the 

 fourth, a small branch of the latter descending to join the 

 anterior primary division of the fifth, and so taking part in the 

 brachial plexus. Each of the anterior primary divisions of the first 

 four cervical nerves is connected with the superior cervical ganglion 

 of the sympathetic by a grey ramus communicans. 



The anterior primary division of the first cervical nerve lies at first 

 in the vertebrarterial groove of the atlas under cover of the verte- 

 bral artery. It then passes forwards in a groove on the outer 

 surface of the superior articular process of the atlas, having the 

 vertebral artery on its outer side. It next emerges between the 

 rectus capitis lateralis muscle (to which it gives a branch) and the 

 rectus capitis anticus minor, and descends in front of the root of 

 the costal process of the atlas to join the ascending branch of the 

 second nerve. From the loop so formed branches are given to the 

 recti antici muscles, major and minor, and one or more branches 

 pass to the hypoglossal nerve, the destination of their fibres being 

 the descendens cervicis and thjiro-hyoid ner\^es. 



