372 



THE MUSCULAE SYSTEM. 



FIG. 332. THE LEFT SERRATUS ANTERIOR 

 MUSCLE. 



The sternoclavicularis is a small separate 

 slip, rarely present, extending beneath the pector- 

 alis major from the upper part of the sternum to 

 the clavicle. 



M. Serratus Anterior. The serratus 

 anterior (O.T. serratus magnus) is a large 

 curved quadrilateral muscle occupying the 

 side of the chest and medial wall of the 

 axilla. It arises by fleshy slips from the 

 lateral aspect of the upper eight and occa- 

 sionally (as in the figure) from nine ribs. 

 The first slip is a double one, arising from 

 the first two ribs and the fascia covering 

 the intervening space (Fig. 332). 



The insertion of the muscle is threefold. 

 (1) The first portion of the muscle (from 

 the first and second ribs) is directed pos- 

 teriorly to be inserted into the costal aspect 

 of the medial angle of the scapula. (2) The 

 next three slips of the muscle (from the 

 second, third, and fourth ribs) are inserted 

 into the vertebral margin of the scapula. 

 (3) The last four slips (from the fifth, sixth, 

 seventh, and eighth ribs) are directed ob- 

 liquely upwards and posteriorly, to be in- 

 serted on the costal aspect of the inferior 

 angle of the scapula (Fig. 333). 



Triceps brachii 



(origin of long 



head) 



The lateral surface of the 

 muscle is partly superficial below 

 the axillary space, on the side wall 

 of the chest, where its slips of 

 origin are seen inter-digitating 

 with those of the obliquus externus 

 abdominis. Higher up it forms 

 the medial wall of the axilla, and 

 is in contact with the pectoral 

 muscles anteriorly and the sub- 

 scapularis posteriorly. Its superior 

 border appears in the floor of the 

 posterior 'triangle, and over it the 

 axillary artery and the cords of 

 the brachial plexus pass in their 

 course through the axilla. The 

 inferior border is oblique, and is 

 in contact with the latissimus dorsi muscle. 

 The muscle may extend higher than usual, so 

 as to be continuous in the neck with the 

 levator scapulae. 



Nerve - Supply. The serratus anterior muscle 

 receives its nerve from the long thoracic nerve, a branch 

 from the anterior trunks of the fifth, sixth, and seventh 

 cervical nerves. After piercing the scalenus medius, 

 the nerve enters the axilla, and supplies branches to the 

 several digitations of the muscle on their superficial 

 surface. The highest fibres of the muscle are supplied 

 by the fifth, the lowest fibres by the seventh, and the 

 intermediate part of the muscle by the sixth cervical 

 nerve. 



Action. The primary action of the muscle is to 

 draw the base of the scapula forwards. This causes 



Deltoid (origin) 



Biceps and coracobrachialis (origin) 

 I Pectoralis minor (insertion) 



Omo-liyoid (origin) 



FIG. 333. MUSCLE-ATTACHMENTS TO THE 

 EIGHT SCAPULA (Anterior Aspect). 



