310 THE MUSCLES 



The internal head arises (1) from the whole of the posterior surface of the 

 humerus between the musculo-spiral groove and the olecranon fossa; (2) on the 

 inner side, from the back of the whole of the internal internuiscular septum; (3) on 

 the outer side, from the back of that portion of the external intermuscular septum 

 which lies below the point where it is jjierced by the nmsculo-spiral nerve. 



Insertion, — (1) The jtosterior \rdi't of the upjjcr svn-face of the olecranon pro- 

 cess; and (2) on either side by aponeuroses which are continuous with the deep 

 fascia on the l)ack of the forearm. 



Structure. — The long head forms a strong fusiform muscular band, arising by 

 tendinous filires which blend with the lower part of the capsule of the shoulder- 

 joint. The surfaces of the band at first look inwards and outwards. The tendon 

 of origin extends for some distance further on the inner than on the outer surface. 

 As the nniscular band passes downwards, it twists upon itself so that what was the 

 internal surface now becomes posterior, and the external surface becoming the 

 anterior is applied to the back of the rest of the muscle. Upon this anterior 

 surface the common tendon of insertion begins as a broad aponeurosis about three 

 inches (8 era.) from the scapula, and receives the fleshy fibres of the long head in 

 penniform fashion chiefly upon its upper border and posterior surface. These 

 fleshy fibres are continued as a thick band along the inner border of this common 

 tendon, and terminate in a blunt point a little below the junction of the middle 

 and lower thirds of the arm. 



The external head arises by fleshy fibres which are inserted in penniform 

 fashion into the outer border of the common aponeurosis. The lowest of these 

 fibres arise from a tendinous arch which bridges over the musculo-spiral nerve. 



The internal head, which is much stronger than the outer, forms a thick 

 triangular sheet, wrapping round the back of the bone, and arising also from the 

 l»ack of the intermuscular septa on either side. The apex of this triangle extends 

 upwards along the lower border of the musculo-spiral groove to a point just below 

 the insertion of the teres major. Its fleshy fibres pass downwards and somewhat 

 backwards to the broad aponeurosis, which, after receiving the fleshy fibres from 

 the outer and long heads, completely covers the whole of the posterior surface of 

 the muscle in the lower third of the arm. A few fleshy fibres of this head are in- 

 serted directly into the olecranon and the adjacent posterior ligament; the slip 

 attached to the ligament is sometimes called the subanconeus. 



The insertion of the tendon into the Ijack part of the upper surface of the ole- 

 cranon is usually separated from the adjacent part of the olecranon and the posterior 

 ligament of the elbow-joint by a small bursa. That part of the aponeurotic contin- 

 uation of the tendon which lies between the olecranon process and the back of the 

 external condyle is by far the stronger. 



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

 seventh and eighth cervical nerves) by means of the musculo-sjnral, which supplies 

 branches to the long and inner heads in the first part of its course; and numerous 

 other branches to the inner and outer heads while passing through the musculo- 

 spiral groove. 



Action. — To extend the elbow-joint, the muscle acting as a lever of the first 

 order, at a very great mechanical disadvantage on account of the short distance 

 which intervenes between the olecranon process and the axis of the elbow-joint ; 

 but at the same time with a great gain in speed and range of movement. Hence 

 this muscle is the chief agent in movements of the arm in which great speed is 

 attained, such as in throwing a stone or striking a blow. The long head has some 

 influence as an adductor of the shoulder-joint ; it has also a supplementary liga- 

 mentous aetion upon that joint similar to that of the coraco-brachialis by holding 

 tlie head of the humerus in close contact with the glenoid cavity. As it passes over 

 both the shoulder- and the ell)ow-joints, the long head enables the powerful abduc- 

 tors of the shoulder to exert a peculiar influence upon the extension of the elbow 

 joint. If it were merely a passive ligament, the long head would extend the elbow 

 whenever the humerus was abducted by the deltoid muscle. As, however, the long 

 head contracts at the same time with tlie deltoid, we have an apparatus by which, 

 so to speak, a double rapidity of extension is secured for the elbow-joint. This 

 attachment, therefore, of tlie long head of the triceps to the scapula is the chief 



