PHRENIC NERVE 695 



one of these crosses over the carotid artery and concurs in the formation of the nerve 

 to the sterno-thyro-hyoideus mentioned above. The ventral branch then becomes 

 superficial by passing between the two parts of the mastoido-humeralis, and divides 

 into posterior auricular and cutaneous branches. The posterior auricular nerve 

 passes upward and forward on the parotid gland parallel with the posterior border 

 of the parotido-auricularis to ramify on the convex face of the external ear. The 

 cutaneous nerve of the neck (X. cutaneus colli) crosses the mastoido-humeralis 

 muscle and turns backward along the course of the jugular vein. On the lower 

 part of the parotid gland it is connected by a twig with the cervical cutaneous 

 branch of the facial nerve. It gives off twigs to the subcutaneous muscles and 

 the skin of the parotid and laryngeal regions and a long branch which passes for- 

 ward in the submaxillary space. 



The third cervical nerve leaves the vertebral canal through the foramen be- 

 tween the second and third cervical vertebrae. Its dorsal branch emerges between 

 two bundles of the intertransversalis muscle accompanietl by a branch of the verte- 

 bral artery, turns upward on the multifidus, antl divides into several branches which 

 radiate on the deep face of the C()m])lexus. It gives branches to these nmscles and 

 to the skin, and a twig which joins the corresponding branch of the fourth nerve. 

 The ventral branch emerges through the intertransversalis below the bundle above 

 which the dorsal branch appears. It gives })ranches to the trachelo-mastoideus, 

 rectus capitis anterior major, longus colli, splenius, and mastoido-humeralis. It 

 also gives off a large cutaneous nerve which jiasses out between the two parts of the 

 mastoido-humeralis and divides into S(n'eral divergent branches. 



The fourth and fifth cervical nerves are distributed in general like the 

 third. Their dorsal branches arc^ united by anastomotic twigs with each other 

 and with those of the third and sixth nerves to form the dorsal cervical plexus. 

 The ventral branch of the fifth nerve often contributes a small twig to the 

 phrenic nerve. 



The sixth cervical nerve has a smaller dorsal branch than the fifth. Its 

 ventral branch is larger and goes in part to the brachial plexus; it supplies twigs 

 to the intertransversales, the longus colli, the mastoido-humeralis, and the cervical 

 parts of the serratus and rhomboideus, furnishes a root of the phrenic nerve, and 

 gives off several considerable subcutaneous branches. One of the latter ramifies 

 on the thick part of the cervical panniculus, to which it gives branches; another and 

 larger branch (N. supraclavicularis) sends twigs to the skin over the shoulder joint, 

 and descends to the skin over the superficial pectoral muscles (Fig. 466). 



The seventh and eighth cervical nerves have small dorsal branches, which 

 ascend between the longissimus and inultifidus, giving twigs to these muscles, the 

 spinalis and semisi:»inalis, the rhomboideus, and the skin. Their ventral branches 

 are very large and concur in the formation of the brachial plexus; that of the 

 seventh nerve contributes the posterior root of the phrenic nerve. 



Phrenic Nerve 

 The phrenic nerve (N. phrenicus) (Figs. 428, 429, 433), the motor nerve to the 

 diaphragm, is formed by the union of two or three roots which cross the superficial 

 face of the scalenus muscle obliquely downward and backward. The chief roots 

 come from the ventral branches of the sixth and seventh cervical nerves. The root 

 derived from the fifth nerve is small and inconstant. The root from the seventh 

 cervical comes by way of the brachial plexus. The course of the nerve is not the 

 same on both sides. On the right side the nerve enters the thorax by passing be- 

 tween the brachial artery and the anterior vena cava. It then courses backward 

 and somewhat downward over the right face of the anterior vena cava, crosses the 

 pericardium, and continues along the posterior vena cava to the diaphragm. In the 

 latter part of its course it is inclosed in a special fold of the right pleura and inclines 



