164: The Development of the Arm in Man 



penetrate directly into the muscle. The last three form a trunk which 

 runs along the lateral surface of the muscle as far as the fourth rib. 



The pedoralis major and minor "^ are united into a common muscle 

 mass, which is well differentiated from the surrounding tissue. It 

 forms a thick oval mass, which extends from the level of the second 

 rib to the proximal portion of the humerus. The greater part of 

 the muscle thus lies anterior to the first rib. As the mass bends 

 towards the humerus it is attached also to the clavicle. So probably both 

 sterno-costal and clavicular portions are present. The median side of 

 the mass bulges towards the coracoid process and represents the minor. 

 Most of the mass shows distinct fibrillation, but toward the humerus 

 this passes into the condensed tissue which is not sharply outlined from 

 the surrounding structures. The position of the pectoral muscle cor- 

 responds to the position of the pectoral premuscle mass in embryo 

 CLXIII. Branches from the median side of the brachial plexus supply 

 the pectoral. Two from the external cord contain fibers from the fifth, 

 sixth and seventh cervical nerves. Two come from the inner cord. 

 Within the muscle complicated anastomoses occur from which fibers 

 spread out in all directions. 



The muscles thus far considered were fairly definite, and, as we have 

 seen, come from quite definite premuscle masses. The remaining mus- 

 cles of the arm are in process of differentiation from the arm premuscle 

 sheath. The exact limits of the individual muscles are almost impos- 

 sible to determine. 



The deltoid muscle extends from the acromion and clavicle and 

 fascia over the infraspinatus to the humerus. It is very closely con- 

 nected with the infraspinatus and only by the difference in the nerve 

 supply can the two be separated. The position of the teres minor is 

 also only indicated by its nerve and not by any line of separation be- 

 tween it and the infraspinatus or deltoid. The origin of part of the 

 deltoid from the acromion and clavicle helps to distinguish some of its 

 fibers, but a short distance from this origin no line of separation can 

 be made between it and the infra- and supraspinatus muscles. Con- 

 densed tissue connects it with the triceps and pectoral muscles. The 

 circumflex nerve supplies this muscle and also sends a branch to fibers 

 which are closely associated with the infraspinatus and probably consti- 

 tute the teres minor muscle. 



That portion of the infraspinatus which lies on the lateral surface 

 of the scapula is fairly distinct except where the deltoid and teres minor 



23 Lewis, Observations on the pectoralis major muscle in man, Johns Hopkins 

 TTosp. Bui., Vol. XII, 1901. 



