MUSCLES PASSLXG TO THE UPPER EXTREMITY 293 



ment; (2) the lower part of the vertebral aponeurosis (see account of Lumbar 

 Fascia, page 408), by which it is attached to the spines of all the lumbar and 

 sacral vertebra?; (8) the posterior third of the outer lip of the crest of the ilium; 

 (4) horizontal lines crossing the outer surface of the last three or four ribs external 

 to their angles (these lines by their lower borders give origin to processes of the 

 external ol)li4ue muscle, which thus interdigitates with the latissimus dorsi); and 

 sometimes (5) the dorsal as})ect of the inferior angle of the scajjula. 



Insertion. — The bottom of the bicipital groove of the humerus, as far upwards 

 as the lesser tuberosity. 



Structure. — Its first and third parts arise by short tendinous fibres. The origin 

 of the second part from the vertebral aponeurosis is by fleshy fibres in a line which 

 descends obliquely downwards and outwards from the last thoracic spine to the 

 back of the crest of the ilium. Its origin from the ribs and scapula is muscular. 

 The fleshy fibres are of nearly equal length, and they converge upon the tendon 

 in such a way that those which arise from the ribs and crista ilii are inserted 

 highest into the humerus, Avhile those which spring from tlie thoracic spine are 

 attached to the lower part of the bicipital groove. The broad sheet wraps round the 

 side of the thorax, and is also folded upon itself, so that the anterior surface at the 

 origin becomes the posterior at the insertion. A groove is thus formed, in which 

 lie the outer border of the scapula and the teres major. The tendon of the teres 

 major is usually attached at the borders to that of the latissimus dorsi by strong 

 connective tissue; but a bursa intervenes between them near their insertion. 



Nerve-supply. — From the posterior cord of the brachial plexus by means of 

 the long subscapular nerve. This is derived from the sixth and seventh cervical 

 nerves, and enters the muscle upon its deep surface in the lower part of the axilla. 



Action. — It draws the humerus backwards, downwards, and inwards; at the 

 same time rotating it inwards. The movement of the arm in swimming is a good 

 example of its action. When the arm is placed close to the side, it will draw the 

 shoulder backwards and downwards. 



Acting from the humerus as a fixed point, it is very important in climbing, as 

 it draws the pelvis and lower part of the trunk upwards and forwards towards the 

 arms. 



By its costal origin it will assist in forced inspiration when the arm is fixed. 



Relations. — Superficially, the trapezius, fasciie, and integument l)ehind, and the 

 pectoralis major, axillary vessels, and branches of the brachial plexus in front; 

 deeply, the rhomboideus major, the vertebral aponeurosis covering the upward 

 continuations of the erector spinoe muscle, the serratus posticus inferior, external 

 intercostals, external obliciue, infraspinatus, serratus magnus, and teres major 

 muscles. • 



Variations. — It varies in the height of its origin from the spinal column, and also in the 

 number of ribs from which it arises. From its axillaiy border shps may cross the axilla to the 

 tendon of the pectoralis major, or may cross the great vessels and nerves to the coracoid process 

 or the deep fascia at the upper part of the arm. _ A slip_ of fascia or muscle may be continued 

 down from its tendon of insertion to the olecranon in association with the triceps. 



ANTERIOR DIVISION OF THE GROUP OF MUSCLES PASSING 

 FROM THE TRUNK TO THE UPPER EXTREMITY 



These muscles are arranged in three layers, of wliicli the first two are formed 

 by muscles which arise from the front of the thorax: viz., the pectoralis major in 

 the first layer, and the subclavius and ])ectoralis minor in the second; the last l)y a 

 single nuiscle, tlie serratus magnus, which takes origin from the side of the thorax. 



Tbe superficial fascia, which covers all these layers, is but moderately supplied 

 with fat, and is continuous Avith that of all the adjacent regions. It lies both over 

 and beneath the mammarv gland, and sends fibrous septa between its lobules. 



In addition to the superficial fascia, there are three other important fasciae in 

 this region. 



