Warren Harmon Lewis 159 



The phrenic nerve arises from the median side of the trunk formed 

 by the IV and V cervical nerves. It does not reach quite to the level 

 of the first rib. See Fig. 6. 



The brachial plexus is formed from the ventral divisions of the IV, V, 

 VI, VII, VIII cervical and I thoracic nerves. The main portion of the 

 plexus forms a continuous sheet of nerve tissue in which only indica- 

 tions of the three cords can be distinguished. The plexus passes later- 

 ally into the arm without any caudal inclination. On reaching the arm 

 it splits into a dorsal and a ventral division. The dorsal division cor- 

 responds to the continuation of the posterior cord. It passes around 

 the dorsal side of the humerus, decreasing rapidly in size and ends in 

 the premuscle sheath near the distal end of the humerus. Most of it 

 represents the musculo-spiral nerve. A small branch, which is probably 

 the circumflex, is given off near its beginning. Fibers from all the 

 spinal nerves forming the plexus can be traced into this dorsal division. 

 The ventral division is partially divided into two parts, which probably 

 represent the outer and inner cords. From the outer arises the supra- 

 scapular nerve, having fibers from the IV, V, and VI cervical. It passes 

 ventral to the scapulo-humeral junction into the arm premuscle sheath. 

 The rest of this outer cord splits into the musculo-cutaneous and the 

 outer head of the median. The musculo-cutaneous passes into the 

 premuscle sheath on the ventral side of the humerus, and the median 

 into the sheath distal to this, reaching as far as the distal end of the 

 humerus. The inner cord terminates in the ulnar nerve, which runs 

 into the premuscle sheath along the median side of the humerus as far 

 as the beginning of the ulna. Branches going into the pectoral pre- 

 muscle mass leave the median side of the plexus, one mostly from the 

 outer and the other two from the inner cord. They correspond to the 

 external and internal anterior thoracics. In Fig. 6 the lengths of the 

 various nerves are indicated. 



Embryo CIX. 



Embryo CIX measures V. B. 10.5 mm. and X. B. 11 mm. in lengtli 

 and is about five weeks old. There is a marked advance over the pre- 

 ceding stage. Cartilage has made its appearance both in the vertebrae 

 and in portions of the arm skeleton. There is considerable difference 

 in the character of the cartilage of the vertebra from that in the arm. 

 The latter seems more advanced and lias more the appearance of true 

 hyaline cartilage. It is possible that the cartilage appears first in the 

 arm, though I have not been able to examine intervening stages to 



