I2QO HUMAN ANATOMY. 



9. The muscular branches are distributed to (a) certain prevertebral muscles 

 and to (6) the genio-hyoid and the infrahyoid muscles. 



a. The rectus capitis anticus major and minor and the rectus capitis lateralis are supplied 

 by a filament arising from the loop between the first and second cervical nerves. The intertrans- 

 versales, the longus colli and a portion of the rectus capitis anticus major receive their supply 

 from the second, third and fourth, and the upper part of the scalenus anticus receives a twig 

 from the fourth cervical nerve. 



b. The genio-hyoid and the four muscles of the infrahyoid group are innervated by the 

 cervical plexus in a rather roundabout manner. From the first and second cervical nerves are 

 given off one or more branches which join the hypoglossal nerve shortly after its appearance in 

 the neck. These fibres for a time form an integral portion of the hypoglossal and eventually 

 escape from it as the nerve to the genio-hyoid, the nerve to the thyro-hyoid and the n. descen- 

 dens hypoglossi (Fig. 1082). The last-mentioned nerve leaves the hypoglossal at the point 

 where the latter crosses the internal carotid artery and then descends in the anterior cervical 

 triangle. In front of, or sometimes within, the carotid sheath it forms a loop of communication, 

 called the hypoglossal loop or ansa cervicalis (ansa hypoglossi) by inosculation with the 

 descending cervical nerve (n. descendens cervicalis) (Fig. 1082). This descending cervical nerve 

 is derived from the second and third cervical nerves and at first consists of two twigs which later 

 unite in front of the internal jugular vein. From this point it passes downward and inward as a 

 single trunk to reach its point of entrance into the ansa hypoglossi. The ansa may be either a 

 simple loop or a plexus and is situated anterior to the carotid sheath at a variable point in the 

 neck. From it branches are given off to the sterno-hyoid, the sterno-thyroid and the posterior 

 belly of the omo-hyoid (Fig. 1076). 



10. The phrenic nerve (n. phrenicus), although an internal muscular branch 

 of the cervical plexus, is of such importance as to merit a separate description. 

 Whilst mainly the motor nerve to the diaphragm, it contains some sensory fibres ; in 

 this connection it may be pointed out that the phrenic is not the only motor nerve 

 to the diaphragm, the lower thoracic nerves aiding in its innervation. The phrenic 

 arises mainly from the fourth cervical nerve but receives additional fibres from the 

 third and fifth (Fig. 1090). It passes down the neck on the scalenus anticus, which 

 it crosses from without inward, and at the base of the neck accompanies that muscle 

 between the subclavian artery and vein. At the entrance to the thorax it passes 

 over the root of the internal mammary artery from without inward and backward, 

 occupying a position behind the sterno-clavicular articulation and the point of junc- 

 tion of the subclavian and internal jugular veins. It then follows a course almost 

 vertically downward, over the apex of the pleura and through the superior and 

 middle mediastina, to the upper surface of the diaphragm. 



The right phrenic (Fig. 1090) is shorter than the left on account of its 

 more direct downward course and the greater elevation of the diaphragm on 

 that side. It crosses the second part of the subclavian artery and accompanies 

 the right innominate vein and the superior vena cava on their lateral aspect. 

 It then passes in front of the root of the lung and finishes its course by de- 

 scending between the lateral aspect of the pericardium and the mediastinal pleura. 

 Nearing the diaphragm it breaks up at the antero-lateral aspect of the quadrate 

 foramen into its terminal branches, a few of which enter the abdomen through 

 this opening. 



The left phrenic (Fig. 1090), having to wind around the left side of the 

 heart and reach the more inferior half of the diaphragm, is longer than its fellow, 

 about one-seventh longer (Luschka). Entering the thorax between the subclavian 

 artery and the left innominate vein it crosses the anterior face of the left vagus nerve 

 and continues its downward course by passing over the left side of the aortic arch. 

 Reaching the middle mediastinum it courses in front of the root of the lung, behind 

 the lower left angle of the pericardium, and descends to the diaphragm between the 

 pericardium and the mediastinal pleura. It breaks up into its terminal branches 

 before arriving at the thoracic surface of the diaphragm, which it enters at a point 

 further from the median line and more anterior than does the ri^lit. 



Branches of the phrenic nerve are : (a) the pleura/, {b} the pcricardiac and 

 f the terminal. 



