100 A MANUAL OF ANATOMY. 



which opens into the lower part of the external jugular 



vein. See page 74. 



The Phrenic Nerve. Figs. 17, 18. 



The phrenic nerve is formed by branches from the third, 

 fourth, and fifth cervical nerves. The fourth is the constant 

 branch, the fifth may be present, but the branch from the 

 third is very inconstant. 



The phrenic descends obliquely across the front of the 

 scalenus anticus muscle, passes between the subclavian 

 artery and vein, and enters the chest over the front of the 

 internal mammary artery. See thorax for rest of course. 



Scalenus Anticus. Fig. 18. 



Origin. From the anterior tubercles of the transverse 

 processes of the third, fourth, fifth, and sixth cervical ver- 

 tebrae. 



Insertion. Into the scalene tubercle upon the upper sur- 

 face and inner border of the first rib. 



Nerve Supply. From the anterior divisions of the fourth, 

 fifth, and sixth cervical nerves close to the intervertebral 

 foramina. 



Action. Both muscles, acting from below, will flex the 

 neck ; one acting, will rotate the neck to the opposite side 

 and flex the neck laterally. Acting from above, the mus- 

 cles will raise and fix the first rib, so aiding in inspiration. 



Scalenus Medius. Fig. 18. 



Origin. From the posterior tubercles of the transverse 

 processes of the lower six cervical vertebrae (sometimes 

 the atlas also). 



Insertion. Into the upper surface of the first rib between 

 the subclavian groove and the tuberosity. 



Nerve Supply. The branches from the posterior divisions 

 of the cervical nerves. 



