168 The Development of the Arm in Man 



The muscle fibers of the extensor groups do not extend as far distally 

 as do those of the flexor mass. 



The Nerves. — The enormous size of the lower cervical nerves at- 

 tracts the attention at once. In the plates and figures they are given in 

 their true proportion to the other structures. The main portion of the 

 brachial plexus has but a very slight posterior inclination. 



A branch from the V cervical supplies the rhomboid muscle mass. 



The V, VI, VII and VIII cervical and I thoraeic nerves unite to form 

 the brachial plexus. The IV cervical does not connect with the plexus. 

 The main portion of the plexus forms a continuous sheet in which 

 indications of the three cords can be seen. The V and VI unite before 

 joining the others and from this union is given off the -suprascapular. 

 It leaves the trunk at right angles and has the appearance of having its 

 proximal end dragged distally to\\'ard the arm by the main portion of 

 the plexus. The VIII and I thoracic unite before joining the plexus. 

 The continuous sheet formed by these five nerves soon splits into a 

 lateral (dorsal) and median (ventral) division. The lateral corresponds 

 to the posterior cord and from it arise the circumflex, subscapular and 

 musculospiral nerves. These nerves take the normal course found in 

 adult and supply the same muscles as in adult. Cutaneous branches are 

 also given ofl^. The median sheet of the plexus quickly divides into 

 several bundles. The anterior one corresponds to the distal end of the 

 external cord. From it are given off the musculo-cutaneous, two 

 branches to the pectoral mass, and one head of the median nerve. 

 The posterior division corresponds to the distal end of the inner 

 cord. From it arise branches to the pectoral mass, the inner head of 

 the median, the ulnar and internal cutaneous nerves. The distal end 

 of the median splits into a peculiar fan-like arrangement of its branches. 

 Both median and ulnar give branches to the deep flexor mass and anasto- 

 mose within the mass. 



I have attempted to trace the origin of the fibers in the main nerves 

 of the arm. The results are given in the following table: 



Cervical. Thoracic. 



Suprascapular V, VI ? 



Subscapularis V, VI, VII 



Long thoracic VII, VIII I 



Anterior thoracics V, VI, VII, VIII I 



Musculo-cutaneous V, VI, VII ? 



Median V, VI, VII, VIII I 



Circumflex V, VI, VII 



Musculospiral V, VI, VII, VIII I 



Ulnar VI?, VII, VIII I 



