SHOULDER, ARM, FOREARM, HAND. POSTERIOR I'.lKT. 355 



in such a way as not to injure the nerve-supply from the musculo-spiral. 

 Now pull these muscles slightly aside and see the supinator hrevis. ( Fig. 247.) 

 Trace it to the following origins and insertions: 



( Origins. ( I ) Lower and lurk part of external condyle ; (2) the external lateral 

 ligament of the elbow-joint; (3) the orbicular ligament; (4) the triangular 

 depression below the lesser M-m<ml cavity o/ the ulna, especially along its pos- 

 terior margin, which forms the upper part of the external border of the ulna. 



lnsertio)i. (I) The back of the neck of the radius ; (2) the anterior and 

 outer surfaces of the radius above and at the upper border of the oblique line. 



You will now find the posterior interosseous nerve issuing from the lower 

 border of the muscle. This nerve supplies all the muscles in this region except 

 the radial group, which were supplied by the musculo-spiral, before this nerve 

 divided into the radial and posterior interosseous. The radial nerve is a branch 

 of the musculo-spiral. 



The radial nerve perforates the deep fascia, between the extensor carpi 

 radialis and supinator longus about three inches above the carpus. Find where 

 the nerve comes through the deep fascia and trace the same to the dorsum of 

 the hand, and to three and one-half fingers. 



The anconeus (Fig. 245) arises from the back part of the outer condyle. 

 It is inserted into the olecranon and posterior surface of the ulna. Remove this 

 muscle from its attachment, turn the same aside, and see the orbicular ligament 

 of the radius and the elbow-joint. Cut through the orbicular ligament and 

 dislocate the head of the radius. 



The region of the shoulder (Fig. 249) shows : 



1. The deltoid muscle and its nerve-supply, the circumflex. 



2. The supraspinatus muscle (Fig. 251) under the trapezius. (Fig. 250.) 



3. The infraspinatus muscle under the deltoid aponeurosis. 



4. The feres minor muscle, from the axillary border of scapula. 



5. The triceps muscle and its three heads. 



6. The feres major muscle, with the latissimus dorsi. 



7. The triangular space and the dorsalis scapula; artery. 



8. The quadrangular space and its circumflex structures. 



9. The circumflex nerve and vessels in the quadrangular space. 

 I o. The anterior and posterior circumflex arteries and veins. 



1 1. The musculo-spiral nerve and the superior profunda artery. 



12. The scapular head of t/ic triceps muscle between the tereals. 



Tlie circumflex quadrangular space (Fig. 249) is bounded : (i) Externally by 

 the surgical neck of the humerus ; (2) internally by the long or scapular head 

 of the triceps ; (3) superiorly, by the teres minor ; (4) inferiorly by the latissimus 

 dorsi and teres major muscles; (5)- contents : posterior circumflex artery, a 

 branch of the axillary, and the circumflex nerve, from the posterior cord of the 

 brachial plexus. 



The triangular space (Fig. 249) is bounded : (i) Externally by the scapular 

 head of the triceps ; (2) superiorly by the teres minor ; (3) inferiorly by the teres 

 major and latissimus dorsi ; (4) contents (Fig. 249) : arteria dorsalis scapulae, a 

 branch of the subscapular artery. 



A second triangular space is bounded (Fig. 249): (i) Externally by the 

 humerus and- outer humeral head of triceps ; (2) internally by the scapular head 

 of the triceps ; (3) superiorly by the teres major and latissimus dorsi ; (4) it 

 contains the musculo-spiral nerve, from the posterior cord of the brachial plexus, 

 and the superior profunda artery, a branch of the brachial ; (5) it transmits the 

 external cutaneous branch of musculo-spiral nerve. 



Dissection. The Deltoid Muscle (Fig. 250). Develop the posterior border 

 of this muscle by cutting freely with the scissors the deltoid aponeurosis between 



