THE HEAD AND NECK. 77 



DESCENDING. (i) Suprasternal supplies the skin over the origins of the sterno- 

 mastoid, sterno-hyoid, sterno-thyroid. (2) Supraclavicular supplies the 

 skin over the thoracic part of the platysma, and over the costal attachments 

 of the scaleni. (3) Supraacromial supplies the skin over the omo-hyoid and 

 over the clavicular and scapular insertions of the trapezius. 



DEEP BRANCHES. 



INTERNAL. (i) Communicating branches to the pneumogastric, hypoglossal, 

 sympathetic. (2) Muscular branches to the rectus capitis anticus major and 

 minor, and rectus capitis lateralis, and longus colli. (3) Commumcans 

 hypoglossi, two cervical branches that unite with the descendens hypoglossi 

 to form the ansa hypoglossal loop, for the supply of the depressor muscles 

 of the hyoid bone. (4) The phrenic a mixed nerve that supplies the 

 diaphragm and the diaphragmatic parts of the pericardium, pleurae, and 

 peritoneum. 



EXTERNAL. (i) Communicating branches to the spinal accessory nerve, in the 



trapezius and under the sterno-cleido-mastoid muscle. (2) Muscular to the 



trapezius, levator anguli scapulae, scalenus medius, and sterno-cleido-mastoid. 



The Brachial or Axillary Plexus. A comprehensive knowledge of this 



plexus, for dissecting-room purposes, must include the following points : 



1. The formation by the union of the anterior primary divisions of the fifth, 

 sixth, seventh, and eighth cervical nerves, and the greater part of the first dorsal 

 or thoracic nerve. 



2. The location of emergence of the above between the scalenus anticus and 

 scalenus medius muscles. 



3. The communications which are with the fourth cervical ; with the second 

 and third thoracic nerves ; with the phrenic nerve ; with the inferior cervical 

 ganglion of the sympathetic. 



4. The relations with the subclavian artery below ; with the clavicle and 

 subclavius muscle in front ; with the serratus magnus and subscapular muscles 

 behind. 



5. The sheath formed by the axillary prolongation of the third layer of deep 

 cervical fascia, investing the plexus and axillary vessels, and finally forming the 

 suspensory or axillary fascia proper. 



6. The formation of three brachial trunks : upper, middle, and lower. The 

 upper by the fusion of the fifth and sixth ; the middle by the seventh ; and the 

 lower trunk by the eighth cervical and the greater part of the anterior primary 

 division of the first dorsal nerve. 



7. The splitting of the brachial trunks into anterior and posterior brachial 

 divisions. 



8. The formation of the posterior cord by the union of the three posterior 

 brachial divisions. 



9. The formation of the outer cord by the union of the anterior brachial 

 divisions of the upper and middle trunks. 



10. The formation of the inner cord by the anterior brachial division of the 

 lower trunk. 



1 1 . The branches of distribution to muscles ; to the skin covering these 

 muscles in general ; to the skin covering the insertions of these muscles in 

 particular ; to articulations where there is motion produced by muscles supplied 

 by the brachial plexus. 



12. The inosculation between sensory nerves and motor nerves peripherally. 



13. Branches given off above the clavicle; branches thrown off below the 

 clavicle, or from the plexus proper. 



