DEEP BRANCHES OF THE CERVICAL PLEXUS. 833 



Deep Branches of the Cervical Plexus. Internal Series. 



The communicating branches consist of several filaments which pass from the 

 loop between the first and second cervical nerves in front of the atlas to the 

 pneumogastric, hypoglossal (see page 825 and Fig. 493) and sympathetic, and a 

 communicating branch between the fourth and fifth cervical. 



Muscular branches supply the Anterior recti and the Rectus lateralis mus- 

 cles ; they proceed from the first cervical nerve and from the loop formed 

 between it and the second. The Longus colli is supplied from the third and 

 the fourth. 



The Communicans Hypoglossi (Fig. 493) consists usually of two filaments, one 

 being derived from the second, and the other from the third cervical. These 

 filaments pass downward on the outer side of the internal jugular vein, cross in 

 front of the vein a little below the middle of the neck, and form a loop with the 

 descendens hvpoglossi in front of the sheath of the carotid vessels (see page 825). 

 Occasionally, the junction of these nerves takes place within the sheath. 



The Phrenic Nerve (internal respiratory of Bell) arises chiefly from the fourth 

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

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

 front of the Scalenus anticus, 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 en 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. At the upper part of the thorax it crosses in front of the arch of 

 the aorta to the root of the lung. 



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

 chest is joined by a fiian.ent 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 is also usually connected by 

 a filament with 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 

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

 From the left nerve filaments pass to join the phrenr' 1 * 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 occipital triangle, and beneath the Trapezius. 



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 supplied sometimes from the third, sometimes 

 the fourth, and occasionally from both nerves. 



