320 THE MUSCLES. 



occupying, with the short extensor, the space comprised between the posterioi 

 border of the scapula and the humerus. 



Structure and Attachments. — The fleshy mass constituting this muscle is 

 formed of very thick fasciculi, among which are found some aponeurotic bands. 

 These fasciculi have their origin on the dorsal angle and the axillary border of 

 the scapula, either directly, or through the medium of two strong fibrous layers, 

 between which they are at first included. They are afterwards directed back- 

 wards and downwards, and converge towards a thick tendon which occupies the 

 posterior and inferior angle of the triangle represented by this muscle. The 

 tendon terminates by attaching itself to the summit of the olecranon, after 

 receiving a great nmnber of fibres from the short extensor, and after gliding, 

 by means of a synovial capsule, over the eminence which serves for its insertion. 



Relations. — The external surface is covered by a thin, fibrous, white-aud- 

 yellow layer, which separates it from the panniculus ; it has near the upper 

 border an excavation into which is received the posterior portion of the long 

 abductor. Its internal face responds to the latissimus dorsi, the adductor of 

 the arm, and the long extensor. Its posterior border is margined by the latter 

 muscle ; the superior follows the axillary border of the scapula, and is attached 

 to it to constitute the fixed insertion of the muscle ; the inferior border responds 

 to the short and middle extensors. 



Action. — It is an extensor of the forearm. 



2. Shokt Extensor of the Forearm (Caput Medium), or External 



Portion of the Triceps (Fig. 179, 8). 



Synonym. — Humero-olecranius extern us — Girard. 



Situation — Direction — Form — Structure. — This muscle is situated between the 

 humerus and the inferior border of the preceding muscle, and is directed 

 obliquely downwards and backwards. It is thick and short, flattened and 

 aponeurotic at its upper extremity, prismatic, and entirely formed of thick 

 parallel muscular fasciculi for the remainder of its extent. 



Attachments. — One of its attachments is on the humerus, to the curved line 

 extending from the deltoid ridge to the base of the articular head (see for this 

 line Fig, 62, above 4), by the short aponeurosis of its superior extremit j— fixed 

 insertion ; the other is to the olecranon, either directly, or through the tendon 

 of the large extensor — movaUe insertion. 



Relations. — The prismatic shape of this muscle offers three faces, which 

 respond : externally, to the two abductors of the arm and to a slight fibrous 

 layer continuous ; above, with that which covers the large extensor ; and below, 

 with the antibrachial aponeurosis ; internally, to the small extensor — from which 

 it is difficult to separate it — to the short flexor of the forearm, and to the anterior 

 extensor of the metacarpus ; superiorly, to the large extensor, which closely 

 adheres to it. 



Action. — An extensor of the forearm. 



3. Middle Extensor of the Forearm (Caput Parvum), or Internal 



Portion of the Triceps (Fig. 181, 8). 



Synonyms. — Humero-olecranius internus — Girard. {Caput parvum — PercivaM.) 



Situation — Direction — Form — Structure. — This muscle is situated at the 

 internal face of the humerus, along the inferior border of the large extensor. 



