MUSCLES OF THE ANTERIOR LIMBS. 313 



acapula, the inner face of the scapular aponeurosis, the supra-scapular fossa, and 

 the anterior border and cervical angle of the scapula— ^?i-<?rf insertion. These 

 fibres, on reaching the inferior extremity of the muscle, form two veiy thick, 

 short, and slightly tendinous branches, united to each other by the enveloping 

 flexor brachii or biceps aponeurosis. The external branch reaches the summit of 

 the external trochanter of the humerus ; the internal is inserted into the corre- 

 sponding part of the internal trochanter — movable insertion. 



Relations. — Outwards, with the scapular aponeurosis, to which its fibres adhere 

 in the most intimate manner ; inwards, with the scapula and the subscapularis 

 muscle ; forwards, with the small pectoral ; and behind, with the scapular spine 

 and the infra-spinatus. The two terminal branches cover and embrace the flexor 

 brachii tendon, and the capsule of the scapulo-humeral articulation. 



Action. — This muscle is an extensor of the humerus, and a tensor of the 

 enveloping flexor brachii aponeurosis. With regard to the articulation of the 

 shoulder, it plays the part of a powerful ligament — a function it shares with 

 the majority of the other scapular muscles. 



5. Infra-spinatus (Postea-spinatus) (Fig. 179, 3, 3'). 



Synonym. — Subacromio-troc 1 1 i teri us — Girard. 



Situation — Form. — Situated, as its name indicates, in the infra-spinatus fossa, 

 this muscle is wide, tliin, and flattened on both sides at its superior extremity, 

 thick and prismatic in its middle, and conoid at its inferior extremity, which is 

 terminated by two short branches — an external and internal. 



Strwture. — The muscular fibres are directed, like the muscle itself, forward 

 and downward ; they are deeply mixed with strong aponeurotic layere. Of the 

 two branches in which it terminates inferiorly, the external is the strongest, and 

 is entirely constituted by a powerful tendon ; the internal is both muscular and 

 aponeurotic. 



Attachments. — All the fleshy fasciculi of this muscle are fixed, either directly, 

 or through the medium of the internal aponeurotic layers : 1. To the whole 

 extent of the infra-spinatus fossa. 2. To the scapular spine and its tuberosity. 

 3. To the cartilage of prolongation of the scapula. 4. To the internal face of 

 the scapular aponeurosis— /j^pf? insertion. The movable insertion of the muscle 

 takes place, on the external tuberosity, by its two terminal branches, the interaal 

 passing within the convexity ; and the strong tendon constituting the external 

 branch (Fig. 179, 3') gliding, by means of a synovial bursa, over the surface of 

 this convexity, and attaching itself to the roughened facet which forms the crest 

 of the external tuberosity. 



Relations. — It is covered by the anterior portion of the long abductor of the 

 arm, and by the scapular aponeurosis. It covere the scapula, its cartilage, the 

 fixed insertion of the large extensor of the forearm, and the short abductor, which 

 adheres to it in the most intimate manner at its superior or aponeurotic portion. 

 Its anterior border responds to the scapular spine and supra-spinatus muscle ; the 

 posterior is bordered by the long abductor of the arm. Its inferior extremity 

 protects, outwardly, the capsule of the scapulo-humeral articulation, and is con- 

 cealed beneath the mastoido-humeralis. 



Action. — The infra-spinatus acts on the humerus as an abductor and outward 

 rotator. 



