370 



THE MUSCULAB SYSTEM. 



Sterno-cleido-mastoid (origin) 



The fibres converge towards the proximal part of the arm, and are inseparably 

 blended at a point half an inch from their insertion into the humerus. The 

 muscle is inserted into (1) the lateral border of the sulcus intertubercularis of 

 the humerus, extending proximally to the greater tubercle and blending, laterally, 

 with the insertion of the deltoid, and medially, with the insertion of the latissimus 

 dorsi (Fig. 336, p. 376); (2) from the proximal border of the insertion a 

 membranous band extends proximally to the capsule of the shoulder- joint, en- 

 veloping at the same time the tendon of the biceps ; and (3) from the distal border 

 a band of fascia passes distally to join the fascia of the arm. 



The arrangement of the fibres of the muscle at its insertion is peculiar. The 

 muscle is twisted on itself, so that the lower (sterno-costal) fibres are directed 



upwards and laterally behind the upper (clavicular) 

 part of the muscle ; in consequence the clavicular part 

 is attached to the humerus more distally than the 

 sterno-costal portion, and is inserted also into the 

 fascia of the arm. The twisting of the fibres is specially 

 found in the inferior sterno-costal fibres of the muscle 

 and the abdominal fibres. These curve upwards 

 behind the superior sterno-costal fibres, and have the 

 highest attachment to the shaft of the humerus, 

 helping to form the fascial expansion which extends 

 upwards over the biceps tendon to the capsule of 

 the shoulder-joint. In this way a bilaminar tendon 

 is produced united along its inferior border; consisting 

 of a superficial lamina formed by the superior sterno- 

 costal fibres, blending for the most part with the- 

 tendon of the clavicular portion ; and a deep lamina, 

 composed of the twisted lower sterno-costal and 

 abdominal fibres. The disposition of the muscular 

 fibres at their insertions is the reason for the applica- 

 tion of the terms " portio attollens " to the clavicular 

 portion, and "portio deprimens" to the sterno-costa 

 and abdominal portions of the muscle. 



Placed superficially, the pectoralis major forms 

 the anterior wall and anterior fold of the axilla. Its 

 superior border is separated from the edge of the 

 deltoid muscle by an interval in which lie the cephalic 

 vein and deltoid branches of the a. thoracoacromialis 

 Its deep surface is in relation with the ribs and inter 

 costal muscles, the costo-coracoid membrane and the 

 structures piercing it, the pectoralis minor, the 

 axillary vessels, and the nerves of the brachial plexus 



Nerve-Supply. The pectoralis major has a double nerve 

 supply, from both anterior thoracic nerves. The lateral anterioi 

 thoracic nerve, derived from the lateral cord of the brachia 

 plexus (C. 5. 6. 7.), divides into two trunks. One pierces the costo-coracoid membrane, and supplie. 

 the clavicular part, and superior portion of the sterno-costal part of the muscle. The other branch 

 communicates over the axillary artery with the medial anterior thoracic nerve, a derivative o 

 the medial cord of the brachial plexus (C. 8. T. 1.). They then supply the pectoralis minor and 

 piercing that muscle, terminate in the lower part of the pectoralis major. 



Action. The pectoralis major draws the arm to the side. The clavicular fibres flex th< 

 shoulder-joint and raise the arm besides drawing it forwards. The sterno-costal and abdomina 

 portions, on the other hand, depress the arm, while drawing it forwards. 



Sternalis Muscle. The sternalis is an occasional muscle placed, when present, parallel t( 

 the sternum upon the sterno-costal origin of the pectoralis major. It has attachments whicl: 

 are very variable both above and below, to the costal cartilages, sternum, rectus sheath, sterno 

 mastoid, and pectoralis major. Its nerve-supply is from one or both of the anterior thoraci< 

 nerves. In certain rare cases it has been said to be innervated by intercostal nerves. It ii 

 present in 4 -4 cases out of 100, and it is slightly more frequent in the male than in the female 

 It has been regarded by different observers as (1) a vestige of the panniculus carnosus, (2) 

 homologue of the sterno-mastoid, or (3) a displaced slip of the pectoralis major. 



Chondroepitrochlearis, Dorsoepitrochlearis, Axillary Arches, Costocoracoideus. On 



Rectus 



abdominis 



(insertion) 



FIG. 330. MUSCLE- ATTACHMENTS TO 

 THE FRONT OF THE STERNUM. 



