PRACTICAL CONSIDERATIONS : AXILLA AND SHOULDER. 579 



Variations. — The portion of the deltoid which arises from the clavicle is subject to con- 

 siderable variation, either being greatly reduced in size or even entirely suppressed, or else 

 being more extensively developed than usual, so that it is in contact or even fused with the 

 clavicular portion of the pectoralis major. It may also be distinctly separated from the remain- 

 der of the muscle, and not infrequently a separation of the acromial and spinal portions may 

 also occur, so that the muscle becomes three-headed. 



Fig. 562. 



Trapezius- 



V 



Spine of scapula 



Acromion — — ^ ^ 



Deltoid, spinal portion— 



Deltoid, acromial portion-^'' \ 





- Stern o-cleido-mastoid 



- Clavicle 



Pectoralis major 



-Deltoid, clavicular 

 portion 



Deltoid muscle viewed from side. 



Accessory bundles of fibres are occasionally found arising from the fascia infraspinata or 

 from some point along the axillary border of the scapula, and either insert with the deltoid 

 {m. basio-deltoideus) or join with the upper part of the muscle, being continued onward as 

 tendinous fibres which pass to the acromion process and lateral extremity of the clavicle (;«. 

 costo-deltoideus). These fibres represent a portion of the deltoid which in the anthropoid apes 

 arises from the borders of the scapula and in some ot the lower mammals forms a distinct 

 muscle. 



PRACTICAL CONSIDERATIONS : THE MUSCLES AND FASCIA OF 



THE AXILLA AND SHOULDER. 



The practical relations of the fascia descending- to the superior borders of the 

 clavicle and scapula have been sufficiently described (page 551). 



Fracture of the Clavicle. — The action of the muscles which move the arm and 

 shoulder and of those attached to the clavicle (page 259) should be considered with 

 reference to the common form of displacement in cases of fracture of the latter bone. 



The acromial fragment, as it moves with the shoulder, is the more markedly 

 affected. It is carried dow7iward by gravity acting on the upper extremity and 

 aided by the two pectoral muscles and the latissimus dorsi. It is drawn inward by 



