1292 HUMAN ANATOMY. 



lateral branch supplies the corresponding part of the left costal portion. The posterior branch 

 (r. phrenicoabdominalis sinister) is distributed to the left lumbar portion of the muscle of the 

 diaphragm and usually either a filament passes to the left semilunar ganglion or several small 

 threads to the cceliac plexus, one of which can be traced to the left suprarenal body. 



The phrenic nerve communicates in the lower part of the neck with the middle or inferior 

 cervical ganglion of the sympathetic. At the inferior aspect of the diaphragm it communicates, 

 on the right side, with the diaphragmatic plexus of the sympathetic and, on the left side, with 

 the semilunar ganglion or the cceliac plexus. 



Variations. The phrenic may receive additional roots from the nerve to the subclavius, 

 the nerve to the sterno-hyoid, the second or the sixth cervical nerve, the n. descendens cervi- 

 calis or the ansa hypoglossi. It may arise exclusively from the nerve to the subclavius or, aris- 

 ing normally, may give a branch to that muscle. It sometimes passes along the lateral border 

 of or pierces the scalenus anticus muscle. Instead of descending behind the subclavian vein it 

 may pass anterior to it or even through a foramen in it. 



The accessory phrenic nerve arises either from the fifth alone or from the fifth and sixth 

 cervical nerves and, entering the thorax either anterior or posterior to the subclavian vein, 

 joins the phrenic at the base of the neck or in the thorax. 



II. The communicating branches of the internal set effect unions with (a) the sympathetic, 

 (d) the vagus and (c) the hypoglossal. 



a. The superior cervical ganglion of the sympathetic or the association cord connecting 

 the superior and middle ganglia sends gray rami communicates to the first, second, third and 

 fourth cervical nerves. 



b. The ganglion of the trunk of the vagus is sometimes connected by means of a tiny 

 nerve with the loop between the first and second cervical nerves. 



c. The hypoglossal nerve receives, just below the anterior condyloid foramen, a good 

 sized branch from the loop between the first and second cervical nerves. This communication 

 furnishes sensory fibres to the hypoglossal nerve which subsequently leaves the latter as its men- 

 ingeal branch ; other spinal fibres leave the twelfth as the n. descendens hypoglossi and as the 

 nerves to the genio-hyoid and thyro-hyoid muscles. 



Practical Considerations. Of the motor nerves of the cervical plexus the 

 phrenic is most commonly the seat of trouble and this may result in or be associated 

 with spasm or paralysis of the diaphragm. The involvement of the diaphragm may 

 be part of a progressive muscular paralysis, as from lead poisoning, or from injuries 

 or diseases of the spine. The nerve may be compressed by tumors or abscesses of 

 the neck, or be injured in wounds of the neck. It passes downward under the sterno- 

 mastoid muscle and on the scalenus anticus, from about the level of the hyoid bone. 

 It is covered and somewhat fixed by the layer of deep fascia covering the scalenus 

 anticus muscle. The clonic variety of spasm, singultus or hiccough, is very common, 

 and is occasionally though rarely dangerous by preventing rest and sleep ; it may 

 complicate apoplexy, peritonitis or chronic gastric catarrh. 



If only one phrenic is paralyzed the disturbance of function is slight and not 

 easily recognized. In a bilateral paralysis, as from alcoholic neuritis, respiration 

 depends almost entirely on the intercostal muscles, since the diaphragm is completely 

 paralyzed. Dyspnoea, therefore, occurs on slight exertion. The epigastrium is 

 depressed rather than prominent and the lower border of the liver is drawn upward. 



The superficial branches of the cervical plexus emerge together through the deep 

 fascia near the middle of the posterior border of the sterno-mastoid muscle, and from 

 this point pass in various directions. The auricularis magnus passes upward and 

 forward over the sterno-mastoid to the ear and parotid gland, the occipitalis minor 

 along the posterior margin of the same muscle to the scalp, and the superficial 

 cervical branch obliquely forward and upward to the submaxillary region. The 

 descending branches are three in number and pass respectively in the direction of 

 the sternum, clavicle and acromion. They give rise to little or no disturbance 

 when wounded. 



THE BRACHIAL PLEXUS. 



The brachial plexus (plexus hrachialis) is a somewhat intricate interlacement of 

 the anterior primary divisions of usually the lower four cervical and first thoracic 

 nerves. To these are sometimes added a branch from the fourth cervical, a branch 

 from the second thoracic, or branches from both of these nerves. The fasciculi form- 



