578 HUMAN ANATOMY. 



ally, so that a triangular interval, the base of which is the humerus, lies between 

 the two muscles. This interval is crossed by the long head of the triceps, which 

 overlies the dorsal surface: of the teres major, and is thus divided into a more medial 

 '.riangidar space, occupied by the dorsal scapular artery, and a more lateral qiiad- 

 rangidar space, through which the posterior circumflex vessels and the circumflex 

 nerve pass. 



Variations. — Considerable variation occurs in the size of the teres major, an increase in 

 the size of that muscle being associated with a diminution of that of the latissimus dorsi, and vice 

 versa. The teres major is, indeed, to be regarded as fundamentally a portion of the latissimus. 



5. SUBSCAPULARIS (Fig. 558). 



Attachments. — The subscapularis is a powerful muscle occupying the ventral 

 (costal) surface of the scapula. It arises from nearly the whole of that surface, with 

 the exception of a small portion near the neck of the bone, some fibres also taking 

 origin from the subscapular fascia. The fibres pass laterally, converging to a strong 

 tendon which is inserted into the lesser tuberosity of the humerus and to a certain 

 extent into the capsule of the shoulder-joint. 



Nerve-Supply. — By the upper and lower subscapular nerves from the fifth 

 and sixth cervical nerves. 



Action. — When the arm is vertical, the subscapularis acts as a powerful inward 

 rotator of the humerus ; when the arm is abducted to a right angle with the body, 

 the muscle serves to draw it forward. 



Relations. — The subscapularis forms a considerable portion of the dorsal wall 

 of the axilla, and is in relation, by its ventral surface, with the axillary vessels and the 

 cords of the brachial plexus, and laterally with the coraco-brachialis and short head of 

 the biceps. Its lower border is in contact with the teres major and with the dorsal 

 scapular vessels and the circumflex nerve. Dorsally it is in contact with the long 

 head of the triceps, and is separated from the neck of the scapula by the large 

 subscapular bursa (bursa m. subscapularis) which frequently is continuous with the 

 synovial cavity of the shoulder-joint. 



Variations. — The subscapularis differentiates in the embryo from the same sheet which 

 gives rise to the teres major and the latissimus dorsi. It is occasionally divided into two or 

 more fasciculi, and sometimes there is separated from its lower portion a small muscle, termed 

 the subscapularis nmior, which arises from the axillary border of the scapula and is inserted into 

 the crest of the lesser tubercle of the humerus and sometimes into the capsule of llie shoulder- 

 joint. 



The sjibscapular fascia is a firm sheet of connective tissue which covers the 

 ventral surface of the subscapularis. It is attached above, medially, and below to the 

 border of the scapula and fades out laterally into the brachial fascia. 



6. Deltoideus (Fig. 562). 



Attachments. — The deltoid is a large triangular muscle which covers the 

 shoulder as with a pad. It arises from the ventral border of the outer third of the 

 clavicle and from the acromion process and lower border of the spine of the scapula. 

 Its fibres pass downward, and converge to be inserted into the deltoid tubercle of the 

 humerus. Where the muscle passes over the greater tuberosity of the humerus a 

 mucous bursa (bursa subdeltoidea ) is interposed between it and that prominence. 



Nerve-Supply.— By the circumflex nerve from the fifth and sixth cervical 

 nerves. 



Action. — To abduct the arm to a position at right angles to the body. Fur- 

 ther abduction is accomplished by a rotation of the scapula by the contraction of 

 the trapezius and the serratus anterior, whereby the lateral angle of the bone is tilted 

 upward. 



Relations. — The deltoid is in relation by its deep surface with the coracoid 

 process and the capsule of the shoulder-joint and with the various muscles attached 

 to or in the neighborhood of these structures. The cephalic vein passes upward 

 along its anterior border. 



