THE SPINAL NERVES. 



Variety. In some cases there is also a communicating- branch from the fourth nerve 

 (fig. 183). 



Phrenic nerve. The diaphragmatic or phrenic nerve (fig. 183, 9 ; 193, 3) 

 passes down through the lower part of the neck and the thorax to its destination. 

 It arises mainly from the fourth cervical nerve, but it also receives, in the majority 

 of instances, an additional root from either the third or the fifth nerve. While 

 descending in the neck, the nerve inclines inwards over the anterior scalenus 

 muscle ; and near the chest it is joined by a filament from the middle or lower 

 cervical ganglion of the sympathetic. 



At the root of the neck each phrenic nerve is placed between the subclavian 

 artery and vein, and crosses over the internal mammary artery from without 

 inwards and backwards. It then takes a nearly vertical course, in front of the 

 root of the lung on each side, and along the side of the pericardium between 

 this and the mediastinal part of the pleura. Near the diaphragm it divides into 

 branches, which pass separately through the muscle, and then diverging from each 

 other, are distributed on the under surface. 



The right nerve has a more direct course than the left, and lies at first along the 

 outer side of the right innominate vein and the superior vena cava. 



The left nerve is somewhat longer than the right, in consequence of the oblique 

 position of the heart and pericardium round which it winds, and also because the 

 diaphragm is lower on this than on the opposite side. It passes into the thorax 

 between the left innominate vein and subclavian artery, and then crosses in front 

 of the left vagus and over the left side of the arch of the aorta before reaching the 

 pericardium. 



Besides the terminal branches supplied to the diaphragm, each phrenic nerve 

 gives on its way through the thora.x filaments to the pleura and pericardium. 

 Luschka describes also twigs from the lower part of the nerve to the peritoneum, 

 and on the right side to the inferior cava and the right auricle of the heart. 



One or two filaments of the nerve of the right side join in a small ganglion with 

 branches to the diaphragm which are derived from the solar plexus of the 

 sympathetic ; and from the ganglion twigs are given to the suprarenal capsule, the 

 hepatic plexus, and the lower vena cava. On the left side there is a junction 

 between the phrenic and the sympathetic nerves near the oesophageal and aortic 

 openings in the diaphragm, but without the appearance of a ganglion. 



According to the observations of Pansini in animals (Arch. ital. de Biologic, x, 1888) the 

 terminal ramifications of the phrenic nerves form in the diaphragm a complicated plexus, in 

 which the three last intercostal nerves also take part, and which contains microscopic ganglia. 

 On sensory fibres in the phrenic nerve, see J. Ferguson, in " Brain," 1891. 



Varieties. The phrenic may receive an accessory root from the second or the sixth cervical 

 nerve (rarely), from the descend ens cervicis or the ansa cervicalis, or from the nerve to the 

 subclavius (frequently). The last may be of considerable size ; and in some instances the 

 whole nerve has been found arising in this way and descending in front of the third part of 

 the subclavian artery and the subclavian vein to the thorax. The accettory phrenic nerve is a 

 filament arising from the fifth, or more rarely the fifth and sixth cervical nerves, and passing 

 either in front of or behind the subclavian vein to join the trunk at the root of the neck or at 

 a variable level in the thorax. The phrenic nerve, having a normal origin, has been observed 

 in different cases lying along the outer border of, or piercing, the scalenus anticus, crossing in 

 front of the subclavian vein, and passing through a ring formed by that vessel. It occasionally 

 gives a branch to the scalenus anticus. 



DEEP BRANCHES : EXTERNAL SERIES. 



Muscular branches. The sterno-mastoid receives a branch from the second 

 cervical nerve. Two branches proceed from the third and fourth nerves to the 

 levator anguli scapulae ; and from the same nerves, as they leave the spinal canal, 



