580 THE NERVOUS SYSTEM. 
The branches to the pleura and pericardium arise as the phrenic nerve 
traverses the mediastinum. The branches to the inferior vena cava, suprarenal 
capsule, and liver arise after communication of the phrenic nerve with the 
diaphragmatic plexus of the sympathetic on the abdominal surface of the diaphragm. 
Communications of the Phrenic Nerve.—1. The phrenic nerve may communi- 
sate with the nerve to the subclavius muscle. 2, It may communicate with the ansa 
hypoglosst, or a branch from it (the nerve to the sterno-hyoid.) 3. It frequently com- 
municates with the cervical part of the sympathetic. 4. It communicates with the solar 
plexus by a junction upon the abdominal surface of the diaphragm with the diaphrag- 
matic plecus on the inferior phrenic artery, in which a small diaphragmatic ganglion is 
found on the right side. From this junction branches are given off to the inferior vena 
cava, suprarenal capsule, and hepatic plexus. 
MORPHOLOGY OF THE CERVICAL PLEXUS. 
The characteristic feature of the cervical plexus is the combination of parts of adjacent nerves 
into compound nerve trunks by the formation of series of loops. The result of the formation of 
these loops is that parts (particularly cutaneous areas) are supplied by branches of more than one 
spinal nerve. 
A. Gutaneous Distribution.—By the combinations of the nerves into loops the discrimination 
of the elements in the upper cervical nerves, corresponding to the lateral and anterior branches 
of a typical thoracic nerve, is made a matter of some difficulty. The second, third, and 
fourth nerves, through the cervical plexus, supply an area of skin extending, laterally, from the 
side of the head to the shoulder; anteriorly, from the face to the level of the nipple. The higher 
nerves supply the upper region (second and third); the lower nerves supply the lower region 
(third and fourth). It is not possible to strictly compare the individual nerves with the lateral 
and anterior branches of a thoracic nerve. A line drawn from the ear to the middle of the 
clavicle separates, however, a lateral from an anterior cutaneous area; and certain of the 
cutaneous nerves fall naturally into one of these two categories. The nerves homologous with 
anterior branches of intercostal nerves are the superficial cervical and the sternal branches of 
the supra-clavicular series ; those homologous with lateral branches are the small occipital and 
acromial branches. The great auricular and clavicular branches are mixed nerves, comprising 
elements belonging to both sets. 
B. Muscular Distribution—The nerves from the cervical plexus, supplying muscles, are 
simpler in their arrangement. They are not generally in the form of loops, and they are easily 
separated into lateral and anterior series. The lateral nerves comprise the branches to the 
rectus capitis lateralis, sterno-mastoid, trapezius, levator anguli scapule. ‘The nerves in the 
anterior series are those to the recti antici, the hyoid muscles, and the diaphragm. 
It is noteworthy that the median muscles—genio-hyoid, thyro-hyoid, sterno-hyoid, omo-hyoid, 
sterno-thyroid, and diaphragm—are continuously supplied by branches from the first five cervical 
nerves: the higher muscles by the higher nerves; the lower muscles by the lower nerves. 
THE BRACHIAL PLEXUS. 
The brachial plexus is formed by the anterior primary divisions of the fifth, 
sixth, seventh, and eighth cervical nerves, along with the greater part of the first 
thoracic nerve. In some cases a slender branch of the fourth cervical nerve is also 
engaged; and the second thoracic nerve, in all cases, also contributes to the inner- 
vation of the arm, through the intercosto-humeral nerve. Im many cases it 
contributes also directly to the plexus, by an intra-thoracic communication with 
the first thoracic nerve. 
Position of the Plexus.—The nerves forming the-brachial plexus appear in 
the posterior triangle of the neck between the scalenus anticus and scalenus 
medius muscles; the plexus is formed in close relation to the subclavian and 
axillary arteries; the nerves emanating from it accompany the artery to the axilla, 
where they are distributed to the shoulder and upper limb. 
Communication with the Sympathetic.— The lower four cervical nerves com- 
municate with the cervical portion of the sympathetic by means of gray rami communi- 
cantes. Two branches arise from the middle cervical ganglion, and join the anterior 
primary divisions of the fifth and sixth nerves. Two arising from the inferior cervical 
ganglion join the seventh and eighth nerves. They reach the nerves either by piercing 
the prevertebral muscles or by passing round the border of the scalenus anticus muscle. 
