764 THE NERVOUS SYSTEM. 



cervical nerve, with a few filaments from the third and a communicating brancn 

 from the fifth. It descends to the root of the neck, running obliquely across the 

 front of the Scalenus anticus, and beneath the Sterno-mastoid, the posterior belly 

 of the Omo-hyoid, and the Transversalis colli and suprascapular vessels. It next 

 passes over the first part of the subclavian artery, between it and the subclavian 

 vein, and, as it enters the chest, crosses the internal mammary artery near its origin. 

 Within the chest it descends nearly vertically in front of the root of the lung and 

 by the side of the pericardium, between it and the mediastinal portion of the pleura, 

 to the Diaphragm, where it divides into branches, which separately pierce that 

 muscle and are distributed to its under surface. 



The two phrenic nerves differ in their length, and also in their relations at 

 the upper part of the thorax. 



The right nerve is situated more deeply, and is shorter and more vertical in 

 direction than the left ; it lies on the outer side of the right vena innominata and 

 superior vena cava. 



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

 to the left side, and from the Diaphragm being lower on this than on the opposite 

 side. It enters the thorax behind the left innominate vein, and crosses in front 

 of the vagus and the arch of the aorta and the root of the lung. In the thorax 

 each phrenic nerve is accompanied by a branch of the internal mammary artery, 

 the comes nervi phrenici. 



Each nerve supplies filaments to the pericardium and pleura, and near the 

 chest is joined by a filament from the sympathetic, and, occasionally, by one from 

 the union of the descendens hypoglossi with the spinal nerves : this filament is 

 found, according to Swan, only on the left side. It frequently receives a filament 

 from the nerve to the Subclavius muscle. Branches have been described as 

 passing to the peritoneum. 



From the right nerve one or two filaments pass to join in a small ganglion with 

 phrenic branches of the solar plexus ; and branches from this ganglion are dis- 

 tributed to the hepatic plexus, the suprarenal capsule, and inferior vena cava. 

 From the left nerve filaments pass to join the phrenic plexus of the sympathetic, 

 but without any ganglionic enlargement. 



Deep Branches of the Cervical Plexus. External Series. 



Communicating Branches. The deep branches of the external series of the 

 cervical plexus communicate with the spinal accessory nerve, in the substance 

 of the Sterno-mastoid muscle, in the posterior triangle, and beneath the Tra- 

 pezius. 



Muscular branches are distributed to the Sterno-mastoid, Trapezius, Levator 

 anguli scapulae, and Scalenus medius. 



The branch for the Sterno-mastoid is derived from the second cervical ; the 

 Trapezius and Levator anguli scapulae receive branches from the third and fourth. 

 The Scalenus medius is derived sometimes from the third, sometimes the fourth, 

 and occasionally from both nerves. 



The Brachial Plexus (Fig. 410). 



The Brachial Plexus is formed by the union of the anterior divisions of the 

 four lower cervical and the greater part of the first dorsal nerves, receiving usually 

 a fasciculus from the fourth cervical nerve, and frequently one from the second 

 dorsal nerve. It extends from the lower part of the side of the neck to the 

 axilla. It is very broad, and presents little of a plexiform arrangement at its 

 commencement. It is narrow opposite the clavicle, becomes broad and forms 

 a more dense interlacement in the axilla, and divides opposite the coracoid 

 process into numerous branches for the supply of the upper limb. The nerves 

 which form the plexus are all similar in size, and their mode of communica- 



