570 HUMAN ANATOMY. 



Action.— When the arm is abducted to a position at right angles to the body, 

 the pectoraUs major will draw the arm forward and at the same time will adduct it. 

 As the arm approaches the vertical position, the adductor action becomes more 

 pronounced and the flexor action less so, and a slight amount of internal rotation 

 appears. When the arm is raised above the level of the shoulder and fixed, the 

 muscle will assist in drawing the trunk upward, as in climbing, and it will also assist 

 in raising the ribs in forced inspiration. 



Variations. — In the lower mammals the pectoralis major is represented by a number ot 

 distinctly separate portions, a condition which may be indicated in man by a more than usual 

 distinctness of the three portions of the muscle and by the occurrence of accessory slips. The 

 sterno-costal and abdominal portions may be greatly reduced or even absent. 



The ni. sferfialis is present in something over 4 per cent, of all cases examined. It is very 

 variable in its development, and consists of fibres which arise anywhere from the third to the 

 seventh costal cartilage, or even from the sheath of the rectus, and extends upward to be aftached 

 to the anterior surface of the sternum, the clavicle, or the tendon of the sterno-cleido-mastoid. 

 Usually the fibres are directed vertically, but sometimes they may have a more or less oblique 

 course. 



The muscle has been variously regarded as a portion of the platysma, a downward pro- 

 longation of the sterno-cleido-mastoid, an upward prolongation of the rectus abdominis, and 

 as a displaced portion of the pectoralis major. The fact that in the majority of cases it is sup- 

 plied by branches from the anterior thoracic nerves indicates clearly its usual derivation from the 

 pectoralis, but it is asserted that in certain cases it received its nerve-supply from the third and 

 fourth intercostal nerves, in which cases it is more probably to be regarded as representing a 

 thoracic portion of the rectus trunk muscles. 



The chondro-epitrochlearis is a slip derived from the pectoralis major which takes its 

 origin from the lower costal cartilages or the abdominal portion of the pectoralis and is inserted 

 into the brachial fascia or the medial epicondjde of the humerus. 



2. Pectoralis Minor (Fig. 560). 



Attachments. — The pectoralis minor lies beneath the pectoralis major. It 

 arises from the outer surface of the third, fourth, and fifth ribs and from the fascia 

 covering the intervening intercostal muscles, and passes obliquely upward and later- 

 ally to be inserted into the coracoid process of the scapula. 



Nerve-Supply. — By branches of the external and internal anterior thoracic 

 nerves from the seventh and eighth cervical and first thoracic nerves. 



Action. — To draw the lateral angle of the scapula downward and forward ; if 

 the scapula be fixed, to raise the ribs to which it is attached. 



Relations. — The pectoralis minor is completely covered by the pectoralis 

 major. It covers the outer surface of the upper ribs and their intercostal spaces, and 

 near its insertion it passes over the middle portion of the axillary vessels and the 

 cords of the brachial plexus. 



3. SuBCLAvius (Fig. 560). 



Attachments. — The subclavius is an almost cylindrical muscle attached at one 

 extremity to the anterior surface of the first costal cartilage and at the other to the 

 under surface of about the middle third of the clavicle. 



Nerve-Supply. — By a special nerve from the brachial plexus from the fifth and 

 sixth cervical nerves. 



Action. — To draw the outer end of the clavicle downward and forward. 



Variations. — The subclavius seems to be the persistent representative of a group of 

 muscles more perfectly developed in the lower mammals and especially in those in which the 

 clavicle is more or less rudimentary. Muscle-bands, which represent portions of the group 

 normally degenerated, are occasionally found in man, and on account of their variable relations 

 have been described under various names. They may all be grouped, however, under three 

 terms, the sterno-chondro-scapularis, the scapulo-cJaviailaris, and the stenio-clavicularis ( Le 

 Double). In the mammals which lack a clavicle — in many Ungulates, for example — a strong 

 muscle-band passes transversely across the upper part of the thorax from the sternum and first 

 costal cartilage to the scapula. This is the sterno-chondro-scapularis, and it occasionally occurs 

 in man as a band arising from the points named, or from either one of them, or from the first 

 rib, and inserting into the coracoid process of the scapula. 



In those mammals which possess a rudimentary clavicle, such as the Rodents, only the 

 terminations of the sterno-chondro-scapular persist, each inserting into the clavicle, and forming 



