DELTOID— SUPRA-SPIXATUS 301 



Insertion. — A rough triangular impression, with the apex downwards, and 

 from two to three inches long, just above the middle of the outer surface of the 

 humerus. 



Structure. — At the front and back part of its origin it arises by short, tendinous 

 fibres which end in parallel muscular bundles. At the middle part, strong fibrous 

 septa three or four in number pass downwards from the acromion process into the 

 substance of the muscle, upon the surface of which their outer edges are visible. 

 The fleshy fibres of this part of the muscle arise both from the acromion and from 

 the surfaces of these septa. The short strong tendon of insertion is prolonged 

 upwards into three fibrous planes, which, as well as the tendon, receive the 

 fleshy fibres of the middle part in multipenniform fasliion. The muscular 

 bundles from the front and back parts of the origin are inserted upon the 

 anterior and posterior surfaces of the tendon of insertion. In front, this tendon 

 is connected with that of the pectoralis major; below, it gives fibres to the 

 external intermuscular septum, and also to the upper part of the brachialis anticus. 



Nerve-supply. — From the posterior cord of the brachial plexus (through the 

 fifth and sixth cervdcal nerves) by means of the circumflex nerve. This nerve 

 enters the deep surface of the muscle by several filaments about half way between 

 the origin and insertion. 



Action. — ^Mlen all its fibres contract together, it will abduct the humerus 

 through a right angle. If the first and second parts act alone, they will flex and 

 aV)duct the arm, as when it is raised to the level of the shoulder and at the same 

 time directed forwards. The posterior and middle portions of the muscle by their 

 contraction will abduct the arm, and at the same time extend it, as when the arm 

 is elevated, and at the same time directed backwards. 



The movements of abduction, or of abduction combined with flexion, are 

 through 90°; of abduction combined with extension, only through 45°. In its 

 action the muscle forms a lever of the third order. The greater advantage which it 

 gains I)y its insertion at a considerable distance from its fulcrum at the shoulder- 

 joint is lost by the extreme oljliquity of its direction. Hence the great thickness 

 and strength of the muscle, and the facility with which this movement is lost by 

 injury or disease. For the proper action of this muscle in elevation of the arm, it 

 is necessary that the scapula should be held firm by means of the serratus magnus. 



Relations. — Superficially, the integument and deep fascia; upon its front 

 border, the pectoralis major, a small artery, and the cephalic vein; deeply, the 

 coracoid process and upper extremity of the humerus, the tendons of the pectoralis 

 minor and major, the short head of the biceps, coraco-brachialis, subscapularis, 

 supra-spinatus, infra-spinatus, teres minor, the long heads of the biceps and triceps, 

 the outer head of the tricei)S, the coraco-clavicular and coraco-acromial ligaments, 

 the circumflex arteries and nerve, and a large bursa which separates the muscle 

 from the greater tuberosity of the humerus and the structures attached to it; below, 

 it comes into contact with the upper part of the brachialis anticus. 



Variations. — The clavicular portion of its origin may reach inwards as far as that of the pec- 

 tcjralis major, and the two muscles ma.v blend along their adjacent borders. Behind, it may 

 receive separate bands from the fascia infra-spinata or the borders of the scapula. Occasionall}' 

 transverse fibres have been found lying in the substance of the muscle close to its acromial origin. 



2. SUPRA-SPIXATUS 



The supra-spinatus — named from its position above the spine of the scapula — 

 is a somewhat fan-shaped, thick triangular sheet. 



Origin. — (1) The inner two-thirds of the supraspinous fossa; (2) the upper 

 surface of the spine of the scapula; and (3) the fascia covering the muscle. 



Insertion. — (1) The upper facet of the greater tuberosity of the humerus; and 

 (2) the capsular ligament of the shoulder-joint. 



Structure. — It arises by fleshy fibres wliich converge upon a tendon which is 

 concealed in the substance of the muscle almost to the point of its insertion. 



Nerve-supply. — From the brachial plexus (through the fifth cervical nerve) 



