THE UPPER LIMB 319 



in pushing, and the strong lower fibres, acting upon the lower angle, 

 rotate the bone so as to elevate the point of the shoulder. In this 

 latter action the muscle is auxiUary to the trapezius. An im- 

 portant use of the muscle is to steady the scapula, more particularly 

 the glenoid cavity, this condition being necessary before the deltoid 

 can elevate the humerus. The muscle by its contraction serves 

 to keep the lower angle of the scapula in contact with the chest 

 wall. When the shoulder is fixed, the lower part of the muscle may 

 elevate the ribs from which it arises. 



When the serratus magnus muscle is paralyzed (in association with the rhom- 

 boid muscles), the condition known as winged scapula is produced. The lower 

 angle and vertebral border of the scapula are displaced backwards when an 

 effort is made to abduct the arm or to thrust it forwards. 



THE SCAPULAR REGION. 



Cutaneous Nerves. — ^The cutaneous nerves of the shoulder are 

 derived from the supra-acromial of the cervical plexus and the cir- 

 cumflex. The supra-acromial nerve, having descended over the 

 outer part of the clavicle and trapezius, divides into numerous 

 branches, which supply the integument over the upper third of 

 the deltoid. The circumflex nerve sends branches through the 

 muscle which supply the integmnent over about its middle third. 

 It also furnishes a special cutaneous branch which turns roimd the 

 posterior border of the muscle a little below the centre, and supplies 

 the integument over its lower third. 



Deep Fascia. — ^The deep fascia is well marked over the infra- 

 spinatus, where it is strongly aponeurotic. It sends septa between 

 that muscle and the teres muscles, and then passes forwards over 

 them to the posterior border of the deltoid, where it splits into two 

 layers which encase that muscle. 



Deltoid — Origin. — (i) The anterior border of the outer third 

 of the clavicle ; (2) the outer border of the acromion process ; 

 and (3) the lower lip of the posterior border of the spine of the 

 scapula. 



Insertion. — ^The deltoid impression on the outer aspect of the 

 humerus, commencing at the centre, and extending upwards for 

 2 inches or more. 



Nerve-supply. — ^The circmnflex nerve, which is a branch of the 

 posterior cord of the brachial plexus, its fibres being derived from 

 the fifth and sixth cervical. 



Blood-supply. — ^The posterior circumflex and acromio- thoracic 

 arteries. 



The muscle, which has very coarse fasciculi, is triangular, the 

 base being upwards. The clavicular portion passes downwards and 

 outwards, the acromial downwards, and the spinal downwards and 

 forwards. 



Action. — ^The acromial portion abducts the arm to the position of 

 a right angle with the trunk ; the clavicular portion draws it for- 

 wards ; and the spinal portion draws it backwards. 



The acromial portion of the muscle, besides arising from the bone. 



