ANATOMY OF A 17.8 MM. HUMAN EMBRYO 67 



right side. The more caudal branch of the ventral ramus of the 

 third cervical nerve extends caudad, and gradually tapers out. 

 Possibly this branch would have joined, eventually, the phrenic 

 nerve (N.phr.). 



The fourth cervical nerve on either side (plates 2 and 3) con- 

 tributes to the formation of the supraclavicular nerves (Nn.s-cL), 

 which extend ventrally along the cephalic border of the jugulo- 

 cephalic vein (plate 5). It also sends a branch to the phrenicus 

 [N.phr.), and to the ventral ramus of the fifth cervical. The root 

 which the phrenic derives from this nerve (plate 2) passes be- 

 tween the two terminal branches of the thyreo-cervical artery. 



The ventral primary division of the fifth cervical nerve gives 

 off the upper root of the long thoracic nerve, and then joins with 

 the sixth cervical to form the upper trunk of the brachial plexus 

 (plate 3). Just before joining, however, the fifth cervical nerve 

 gives off its phrenic root which, extending caudad, dorsal to the 

 thyreo-cervical artery (plate 2), meets the branch from the fourth 

 cervical nerve. Each phrenic nerve (N.phr.), thus formed, ac- 

 companies the internal mammary artery {A.mam.i.) dorso- 

 ventrally around the caudal border of the terminal portion of the 

 subclavian vein {V.scl., plate 5). The nerve here leaves the 

 artery, and at first extends in the somatopleure along the lateral 

 side of the anterior cardinal vein, then between this vein and the 

 pleural cavity, and finally in the pleuro-pericardial membrane, 

 lateral to the common cardinal. The nerves ultimately reach 

 the anlage of the diaphragm on either side of the common hepatic 

 vein (the right phrenic being for some distance in close relation 

 to the latter). 



The ventral primary division of the sixth cervical nerve, dorsal 

 to where it is joined by that of the fifth, gives off the middle root 

 of the long thoracic nerve. 



The ventral primary division of the seventh cervical nerve 

 furnishes the lower or caudal root of the long thoracic nerve, and 

 becomes the middle trunk of the brachial plexus. 



The ventral primary division of the eighth cervical and that 

 of the first thoracic nerves unite to form the lower trunk of the 

 brachial plexus. 



