300 THE MUSCLES 



the deltoid to elevate the arm are unsuccessful, and only cause the lower angle of 

 the scapula to project from the back of the thorax. A\'hen the arm has been ele- 

 vated to its full extent by the deltoid, that is, through about a right angle, the 

 rotation of the sca})ula due to the serratus magnus and trapezius will produce a 

 further elevation through another riglit angle, and so place the arm in a vertical 

 position. One of the results of the action of this muscle is to keep the lower angle 

 of tlie scapula in close contact with the wall of the thorax. When the muscle is 

 paralvsed, the posterior borders and the inferior angles of the scapulae project back- 

 wards from the thorax like small wings {iycapidx alatx). 



Acting from its scapular insertion, the muscle tends to draw the front of the 

 chest towards the scapula, e.g. when it supports the thorax in crawling on the 

 hands and knees. Judging from the direction in which its fibres are inserted into 

 the ribs, most of them can have little if any action in elevating the ribs, as in forced 

 inspiration. 



Relations. — Superficially, the pectoralis major and minor, the subscapularis 

 and latissimus dorsi, the subclavian and axillary vessels and the brachial plexus; 

 deeply, the external intercostal muscles, and serratus jDOsticus superior. 



Variations. — It may arise as low as the tenth rib; and above, it may receive slijis from the 

 cervical transverse processes, or the levator auguli scapulae. Part or the whole of the muscle may 

 be deficient. 



The Second Group comprises the muscles which pass from the scapula to the 

 upper limb, and move the upper arm. Some of the most important adductors, 

 flexors, and extensors belong to the first group; but abduction and rotation in both 

 directions are provided for by the second group. 



MUSCLES WHICH PASS FROM THE SCAPULA TO THE 



UPPER LIMB 



These are nine in number, viz. the deltoid, supra-spinatus, infra-spinatus, teres 

 minor, subscapularis, teres major, coraco-brachialis, biceps, and triceps. The two 

 last, however, as they act chiefly upon the forearm, will not be described in this 

 group. 



The superficial fascia covering the muscles contains but little fat. The deep 

 fascia is thin where it covers the front of the deltoid, and is continuous with that 

 whicli invests the pectoralis major and the axillary fascia. Behind, it is thicker, 

 especially where it covers in the lower part of the infra-spinatus. Below, it lilends 

 with the deep fascia of the upper arm. Above, it is connected with the clavicle and 

 scajnila above the upper attachment of the deltoid. 



It is also connected, either directly or by strong intermuscular septa, with the 

 vertebral and axillary borders of the scapula. 



Processes from it cover the dee]ier surface of the deltoid, and form sheaths to all 

 the muscles of this group. 



1. DELTOID 



The deltoid muscle (figs. 262 and 264) — named from its resemblance to the 

 Greek letter deltn when inverted — is a very thick triangular sheet, with the apex 

 directed downwards, and with its ])lane curved upon itself from before backwards, 

 so as to wrap round the front, outer side, and back of the upjicr end of the 

 humorus. 



Origin. — (1) The anterior border and adjacent part of the u])per. surface of the 

 outer third of the clavicle; (2) the outer border and adjacent part of tlie ujijier 

 surface of the acromion; (3) the lower border of the spine of the scai)ula and 

 the fascia covering the infra-si)iiiatus muscle, near the vertebral border of the 

 scapula. 



