POSTERIOR CERVICAL PLEXUS. 413 



The Phrenic nerve (internal respiratory of Bell) is formed by filaments 

 from the third, fourth, and fifth cervical nerves, receiving also a branch 

 from the sympathetic. It descends to the root of the neck, resting upon 

 the scalenus anticus muscle, then crosses the first portion of the subclavian 

 artery, and enters the chest between it and the subclavian vein. Within 

 the chest it passes through the middle mediastinum, between the pleura 

 and pericardium, and in front of the root of the lung to the diaphragm to 

 which it is distributed, some of its filaments reaching the abdomen through 

 the openings for the oesophagus and vena cava, and communicating with 

 the phrenic and solar plexus, and on the right side with the hepatic plexus. 

 The left phrenic nerve is rather longer than the right, from the inclination 

 of the heart to the left side. 



POSTERIOR CERVICAL NERVES. The posterior division of the first cer- 

 vical nerve (sub-occipital), larger than the anterior, escapes from the ver- 

 tebral canal through the opening for the vertebral artery, lying posteriorly 

 to that vessel, and emerges into the triangular space formed by the rectus 

 posticus major, obliquus superior, and obliquus inferior. It is distributed 

 to the recti and obliqui muscles, and sends one or two filaments down- 

 wards to communicate with the second cervical nerve. The posterior 

 branch of the second cervical nerve is three or four times greater than the 

 anterior branch, and is larger than the other posterior cervical nerves. 

 The posterior branch of the third cervical nerve is smaller than the preced- 

 ing, but larger than the fourth ; and the other posterior cervical nerves go 

 on progressively decreasing to the seventh. The posterior branches of 

 the fourth, fifth, sixth, seventh and eighth nerves pass inwards between 

 the muscles of the back in the cervical and upper part of the dorsal region, 

 and reaching the surface near the middle line, are reflected outwards, to 

 be distributed to the integument. The fourth and fifth are nearly trans- 

 verse in their course, and lie between the semispinalis colli and complexus. 

 The sixth, seventh, and eighth are directed nearly vertically downwards ; 

 they pierce the aponeurosis of origin of the splenius and trapezius. 



POSTERIOR CERVICAL PLEXUS. This plexus is constituted by the suc- 

 cession of anastomosing loops and communications which pass between 

 the posterior branches of the first, second, and third cervical nerves. It 

 is situated between the complexus and semispinalis colli, and its branches 

 are the 



Musculo-cutaneous, Occipitalis major. 



The Musculo-cutaneous branches pass inwards between the complexus 

 and semispinalis colli to the ligamentum nuchas, distributing muscular 

 filaments in their course. They then pierce the aponeurosis of the trape- 

 zius and become subcutaneous, sending branches outwards to supply the 

 integument of the posterior aspect of the neck, and upwards to the poste- 

 rior region of the scalp. 



The Occipitalis major is the direct continuation of the second cervical 

 nerve ; it ascends obliquely inwards, between the obliquus inferior and 

 complexus, pierces the complexus and trapezius after passing for a short 

 distance between them, and ascends upon the posterior aspect of the head 

 between the integument and occipito-frontalis, in company with the occi- 

 GO 



