166 The Development of the Arm in Man 



The hracJiialis muscle is closely attached to the distal one-half of the 

 hmnerns over the anterior and median surfaces. It is also closely at- 

 tached to the overlying biceps muscle and it is impossible to determine 

 just the line between the two or between it and the brachioradialis 

 muscle. It is also impossible to determine the exact line between the 

 muscle and the underlying perichondrium. It is closely associated with 

 the triceps on one side and the deltoid on the other. The main por- 

 tion of the muscle is fibrillated and is inserted into the ulna by condensed 

 tissue, which is closely associated with the, flexor mass of the forearm. 

 The musculo-cutaneous nerve gives off a large branch which has a 

 brush-like endins; within the muscle. 



Spinal accessoiy 



Fig. 11. Outline of the arm region of embryo CIX, median view, from 

 Plate V. Bardeen and Lewis, Vol. I, No. 1, this Journal. X 15 diameters. 



The flexor muscle mass of the forearm forms a thick layer over the 

 median surface of the ulna, radius, carpus and proximal end of the 

 metacarpus. It is with considerable difficulty that I have separated this 

 mass into two layers. The superficial layer is smaller in extent and lies 

 in the proximal region of the forearm. It is connected with the radial 

 portion of the forearm by a condensed tissue mass and distally fuses with 

 the deep layer to become continuous with the condensed tissue of the 

 digits. The median nerve passes through the proximal portion and 

 then comes to lie between the two layers. From its position and rela- 

 tion to the median nerve I believe this to be the layer from which the 

 flexor carpi radialis, flexor sublimis digitorum, pronator teres and pal- 

 niaris long-us muscles differentiate. Branches from the median nerve 

 supply this layer. Both layers arise partly from the inner condyle of 

 the humerus, and are continuous more or less with the muscles of the 

 upper arm. The deep layer is closely attached to the perichondrium of 



