PHRENIC NERVE. o79 
The main part of the trunk accompanies the hypoglossal and separates from it succes- 
sively in three nerves—the descendens hypoglossi, and the nerves to the thyro-hyoid 
and genio-hyoid muscles. ‘he portion of the nerve which remains accompanies the 
hypoglossal to the muscles of the tongue. It is probable that no part of the hypo- 
glossal nerve itself is concerned in the formation of these three branches. ‘The 
| descending branch of the hypoglossal (1. descendens hypoglossi) descends in front of 
— the common carotid artery, and is joined in the anterior triangle of the neck by 
| the descending cervical nerve, to form the ansa hypoglossi, from which the infra- 
| hyoid muscles are innervated. (The descending branch of the hypoglossal, in 
- some cases, arises from the pneumogastric nerve.) 
| Muscular Branches. — The muscles supplied by the internal branches of the 
) 
| 
| 
plexus are the prevertebral muscles, the genio-hyoid and infra-hyoid muscles, and 
the diaphragin. 
(a) Prevertebral Muscles.—1. From the loop between the first and second 
cervical nerves a small branch arises, for the supply of the rectus capitis lateralis 
and the recti capitis antici (major al minor). 2. From the second, third, and 
fourth nerves small br inches supply the intertransversales, longus colli, and rectus 
capitis anticus major. 3. From the fourth nerve a branch arises for the upper part 
_ of the scalenus anticus. 
| ()) Genio-hyoid and Infra-hyoid Muscles.—The descending cervical nerve (n. 
_ cervicalis descendens) is formed in front of the internal jugular vein by the union 
_ of two slender trunks from the second and third cervical nerves. It forms a loop 
of communication in front of the carotid sheath with the descending branch of the 
hypoglossal nerve (derived ultimately from the first two cervic: al nerve s). This 
loop of communication is called the ansa hypoglossi. It is often plexiform; and 
from it branches are given to the sterno-hyoid, sterno-thyroid, and omo-hyoid 
muscles. The nerve to the sterno- hyoid muscle is often continued behind the 
sternum, to joi in the thorax with the phrenic nerve or the cardiac plexus. 
The thyro-hyoid muscle and the genio-hyoid muscle are supplied by branches of 
the hypoglossal nerve, which are traceable back to the communication between the 
hypoglossal and the first two cervical nerves. 
The anterior muscles in immediate relation to the middle line of the neck, 
between the chin and the sternum, are thus continuously supplied by the first 
three cervical nerves. The hypoglossal is the nerve of the muscles of the tongue, 
and it is not certain that it contributes any fibres to the above-named muscles. 
(¢) Diaphragm.—The phrenic nerve supplies the diaphragm. 
PHRENIC NERVE. 
The phrenic nerve (n. phrenicus) is derived mainly from the fourth cervical 
nerve, reinforced by roots from the third (either directly or through the nerve 
to the sterno-hyoid) and fifth (either directly or through the nerve to the 
subclavius muscle). It passes downwards in the neck upon the scalenus 
anticus muscle; at the apex of the thorax it passes between the subclavian 
artery and vein, and traverses the mediastinum to reach the diaphragm, lying 
in the middle mediastinum between the pericardium and pleura, and in front of 
the root of the lung. In its course it presents certain differences on the two 
sides. In the neck, on the left side, it crosses the first part of the subclavian 
artery ; on the right side it, crosses the second part. In the superior mediastinum, 
on the left side, it “lies between the left subclavian and carotid arteries, and crosses 
the pheumogastric nerve and the aortic arch. On the right side it accompanies the 
innominate vein and superior vena cava, and is entirely separate from the pneumo- 
gastric nerve. The left nerve is longer than the right, owing to the position of 
the heart and the left half of the diaphragm. The right nerve sends fibres along the 
inferior vena cava through the foramen quadratum. Reaching the diaphragm the 
nerve separates into numerous branches for the supply of the muscle; some 
enter its thoracic surface (sub-pleural branches), but most of the fibres supply 
it after piercing the muscle (sub-peritoneal branches). 
The branches of the phrenic nerve are—1l. Muscular (to the diaphragm) ; 
pleural; 3. pericardiac ; 4. inferior vena caval; 5. capsular; and 6. hepatic. 
