580 THE CERVICAL NERVES. 



it is joined by a filament of the fcympathetic, and sometimes also by 

 another filament derived from the fifth and sixth cervical nerves. 



As it enters the thorax each phrenic nerve is placed between the sub- 

 clavian artery and vein, and crosses over the internal mammary artery 

 near the root. It then takes nearly a straight direction, 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 dia- 

 phragm it divides into branches, which separately peneti'ate the fibres 

 of that muscle, and then diverging from each other, are distributed on 

 the under surface. 



The rifjlit nerve is placed more deeply than the left, and is at first 

 directed along the outer side of the right innominate vein, and the 

 descending vena cava. 



The nerve of the left side is a little longer than that of the right, in 

 consequence of the oblique position of the pericardium round which it 

 winds, and also because of the diaphragm being lower on this than on 

 the opposite side. This nerve crosses in front of the arch of the aorta 

 and the pulmonary artery before rea<ching the pericardium. 



Besides the terminal hrayicJiea supplied to the diaphrag-m. each phrenic nerve 

 gives filaments to the pleura and pericardiiTm ; and receives sometimes an offset 

 from the union of tlie descendens noni with the cervical nerves. Swan notices 

 this union as occuning' only on the left side. Luschka describes twigs from the 

 lower part of the nerve to the peritoneum, 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 hejiatic plexus, and the lower vena cava. On the left side there is a junction 

 between the phrenic and the sj-mpathetic nerves near the oesophageal and aortic 

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



IV. Deep Branches : External Series. 



Muscular branches. — The sterno-mastoid receives a branch from 

 the second cervical nerve. Two branches proceed from the third nerve 

 to the levator anguli scapula? ; and from the third and fourth cervical 

 nerves, as they leave the spinal canal, branches are given to the middle 

 scalenus muscle. Further, the trapezius has branches prolonged to it ; 

 and thus, like the sterno-mastoid, this muscle receives nerves from both 

 the spinal accessory and the cervical plexus. 



Connection n-\th the q)inal accessor;/ nerve. — In the substance of the sterno- 

 mastoid muscle, this nerve is connected with the branches of the cervical plexus 

 furnished to that muscle. It is also connected with the branches distributed to 

 the trapezius — the imion between the nerves being beneath the muscle, and 

 having the appearance of a plexus ; and with another branch of the cervical 

 plexus in the interval between the two muscles. 



Summary of the cervical plexus. — From the cervical plexus are 

 distributed cutaneous nerves to the back of the head, to part of the ear 

 and face, to the anterior half of the neck, and to the upper part of 

 the trunk. The muscles supplied with nerves from the plexus are the 

 sterno-mastoid. the platysmo, and the lower hyoid muscles in part; 

 the anterior recti capitis, the levator anguli scapulae, the trapezius, 

 the scalenus medius, and the diaphragm. By means of its branches 



