640 THE CERVICAL NERVES AND PLEXUS. 



are two of these nerves, or, in some cases, a greater number. They arise 

 from the third and fourth cervical nerves, and descend in the interval 

 between the sterno-mastoid and the trapezius muscles. As they approach 

 the clavicle, the nerves are augmented to three or more iii number, and are 

 recognised as internal, middle, and posterior. 



a. The internal (sternal) branch, which is much smaller than the rest, ramifies over 

 the inner half of the clavicle, and terminates near the sternum. 



b. The middle branch, lying opposite the interval between the pectoral and deltoid 

 muscles, distributes some twigs over the fore part of the deltoid, and others over 

 the pectoral muscle. The latter join the small cutaneous branches of the intercostal 

 nerves. 



c. The external or posterior branch (acromial) is directed outwards over the acro- 

 mion, and the clavicular attachment of the trapezius muscle, and ends in the integu- 

 ment of the outer and back part of the shoulder. 



III. DEEP BRANCHES : INNER SERIES. 



CONNECTING BRANCHES. 



The cervical plexus is connected near the base of the skull with the 

 trunks of the pneumo-gastric, hypoglossal, and sympathetic nerves, by 

 means of filaments intervening between those nerves and the loop formed 

 by the first two cervical nerves in front of the atlas (p. 637). 



MUSCULAR BRANCHES. 



Branches to the anterior recti muscles proceed from the cervical nerves 

 close to the vertebrae, including the loop between the first two of these 

 nerves. 



Two branches to the ansa hypoglossi, one from the second, the other from 

 the third cervical nerve, descend over or under the internal jugular vein, to 

 form a loop of communication with the rarnus descendens noni, and aid in 

 the supply of the muscles below the hyoid bone (p. 6 20). 



PHRENIC NERVE. 



The diaphragmatic or phrenic nerve passes down through the lower part 

 of the neck and the thorax to its destination. 



It commences from the fourth cervical nerve, and receives usually a fasci- 

 culus from the fifth. As it descends in the neck, the nerve is inclined 

 inwards over the anterior scalenus muscle ; and near the chest it is joined 

 by a filament of the sympathetic, and sometimes also by another filament 

 derived from the fifth and sixth cervical nerves. 



As it enters the thorax each phrenic nerve is placed between the sub- 

 clavian artery and vein, and crosses over the internal mammary artery near 

 the root. It then takes nearly a straight direction, in front of the root of 

 the lung on each side, and along the side of the pericardium, between 

 this and the mediastinal part of the pleura. Near the diaphragm it divides 

 into branches, which separately penetrate the fibres of that muscle, and then 

 diverging from each other, are distributed on the under surface. 



The right nerve is placed more deeply than the left, and is at first directed 

 along the outer side of the right innominate vein, and the descending vena 

 cava. 



