THE POSTERIOR HUMERAL REGION. 389 



septum: the fibres from this origin converge toward the common tendon of 

 insertion. 



The internal head arises from the posterior surface of the shaft of the humerus, 

 below the groove for the musculo-spiral nerve; commencing above, narrow and 

 pointed, below the insertion of the Teres major, and extending to within an inch 

 of the trochlear surface : it also arises from the internal border of the humerus, 

 and from the back of the whole length of the internal and lower part of the 

 external intermuscular septa. The fibres of this portion of the muscle are 

 directed, some downward to the olecranori, whilst others converge to the common 

 tendon of insertion. 



The common tendon of the Triceps commences about the middle of the back part 

 of the muscle : it consists of two aponeurotic laminae, one of which is subcutaneous 

 and covers the posterior surface of the muscle for the lower half of its extent ; the 

 other is more deeply seated in the substance of the muscle : after receiving the 

 attachment of the muscular fibres, they join together above the elboAv, and are 

 inserted, for the most part, into the back part of the upper surface of the olecranon 

 process ; a band of fibres is, however, continued downward, on the outer side, 

 over the Anconeus, to blend with the deep fascia of the forearm. A small bursa, 

 occasionally multilocular, is situated on the front part of this surface, beneath the 

 tendon. 



The long head of the Triceps descends between the Teres minor and Teres 

 major, dividing the triangular space between these two muscles and the humerus 

 into two smaller spaces, one triangular, the other quadrangular (Fig. 232). The 

 triangular space contains the dorsalis scapulae vessels ; it is bounded by the Teres 

 minor above, the Teres major below, and the scapular head of the Triceps 

 externally : the quadrangular space transmits the posterior circumflex vessels 

 and the circumflex nerve ; it is bounded by the Teres minor above, the Teres 

 major below, the scapular head of the Triceps internally, and the humerus exter- 

 nally. 



Relations. By its posterior surface, with the Deltoid above : in the rest of its 

 extent it is subcutaneous ; by its anterior surface, with the humerus, musculo- 

 spiral nerve, superior profunda vessels, and back part of the elbow-joint. Its 

 middle or long head is in relation, behind, with the Deltoid and Teres minor ; in 

 front, with the Subscapularis, Latissimus dorsi, and Teres major. 



The Subanconeus is a name given to a few fibres from the under surface of the 

 lower part of the Triceps muscle, which are inserted into the posterior ligament 

 of the elbow-joint. By some authors it is regarded as the analogue of the Sub- 

 crureus in the lower lirnb, but it is not a separate muscle. 



Nerves. The Triceps is supplied by the seventh and eighth cervical nerves 

 through the musculo-spiral nerve. 



Actions. The Triceps is the great extensor muscle of the forearm, serving, 

 when the forearm is flexed, to extend the elbow-joint. It is the direct antagonist 

 of the Biceps and Brachialis anticus. When the arm is extended the long head 

 of the muscle may assist the Teres major and Latissimus dorsi in drawing the 

 humerus backward and in adducting it to the thorax. The long head of the 

 Triceps protects the under part of the shoulder-joint, and prevents displacement 

 of the head of the humerus downward and backward. The Subanconeus draws 

 up the posterior ligament during extension of the forearm. 



Surgical Anatomy. The existence of the band of fibres from the Triceps to the fascia of 

 the forearm is of importance in excision of the elbow, and should always be carefully preserved 

 from injury by the operator, as by means of these fibres the patient is enabled to extend the 

 forearm, a movement which would otherwise mainly be accomplished by gravity ; that is to say, 

 allowing the forearm to drop from its own weight. 



III. MUSCLES AND FASCLE OF THE FOREARM. 



Dissection. To dissect the forearm, place the limb in the position indicated in Fig. 228 ; 

 make a vertical incision along the middle line from the elbow to the wrist, and a transverse 

 incision at the extremity of this ; the superficial structures being removed, the deep fascia of 

 the forearm is exposed. 



