368 THE MUSCULATURE 



Variations. — Aside from its frequent fusion with the infraspinatus, there has also been 

 reported an isolation of a special fasciculus to the subtubercular attachment. 



The infraspinatus (fig. 363). — Origin. — From the vertebral three-fourths of the infra- 

 spinous fossa, from the under surface of the spine, from the enveloping fascia and from inter- 

 muscular septa between it and the two teres muscles. 



Structure and insertion. — The fibre-bundles converge toward the lateral angle of the scapula 

 to be attached to a deep-seated tendon which is adherent to the capsule of the joint and is 

 attached to the middle facet of the great tubercle. The fibre-bundles arising from the inferior 

 surface of the spine and the fascia near this form a distinct fasciculus which descends on and 

 covers the tendon of insertion. 



Nerve-supply. — From the suprascapular nerve, which passes beneath the supraspinatus 

 muscle and enters the deep surface of the infraspinatus in the lateral part of the midde third 

 of its upper margin. From here rami spread out toward the vertebral border of the muscle 

 and toward the humeral insertion. The nerve fibres are derived from the fifth and sixth 

 cervical nerves. 



Actioji. — This muscle is the chief lateral rotator of the arm, a movement that can be 

 carried through 90°. The upper part of the muscle is an abductor, the lower part an adductor 

 of the arm. The muscle is also a flexor. 



Relations. — The deltoid and trapezius, and sometimes the latissimus dorsi muscles, cover 

 a portion of the dorsal surface. Over most of it extends the complex fascia described above. 

 Laterally it adjoins the teres minor and major muscles. Under the muscle lie the transverse 

 (suprascapular) and circumflex (dorsal) scapular vessels. 



Variations. — These are rare, aside from a greater or less independence of the bundles arising 

 from the spine and a greater or less complete fusion with the teres minor. A fasciculus has 

 been seen extending to the muscle from the deltoid. 



The supraspinatus (fig. 363). — Origin. — Fleshy from the medial two-thirds of the supra- 

 spinous fossa and from the deep surface of the enveloping fascia near the vertebral end. 



Structure and insertion. — The fibre-bundles converge upon a deep-seated tendon nearly to its 

 attachment into the highest of the three facets on the great tubercle of the humerus. 



Nerve-supply. — Two branches from the suprascapular nerve enter the middle third of the 

 deep surface of the muscle. The nerve fibres are derived from the fifth cervical nerve. 



Action. — It aids the deltoid in abducting the arm. It is also a weak lateral rotator and 

 flexor. It keeps the head of the humerus in place during abduction of the arm. 



Relations. — The muscle is covered by the trapezius, the acromion, and the coraco-acromial 

 ligament. Beyond the base of the spine of the scapula it comes into contact with the infra- 

 spinatus muscle. Beneath the muscle pass the suprascapular nerve and transverse scapular 

 (suprascapular) vessels. 



Variations. — The muscle shows slight variations. Its tendon may be fused with that of 

 the infraspinatus. Its belly may be reinforced by fibre-bundles from the coraco-acromial 

 ligament. 



The latissimus dorsi (figs. 355, 356, 387, 388). — Origin. — (1) From an aponeurosis at- 

 tached to the spines and interspinous ligaments of the five or six last thoracic and the upper 

 lumbar vertebrae, to the lumbo-dorsal fascia, and to the posterior third of the external lip of the 

 crest of the ilium; (2) from the external surface and upper margin of the last tlu-ee or four ribs 

 by muscular slips which interdigitate with those of the external oblique. In the lumbar region 

 the aponeuroses of the right and left muscles are connected by fibrous fasciculi which cross the 

 mid-dorsal line above the supraspinous ligament. 



Structure and insertion. — From this extensive area of the origin fibre-bundles converge 

 toward the tendon of insertion. In the region of the dorsal wall of the axillary fossa the muscle 

 is concentrated into a thick, ribbon-like band which winds about the teres major and passes to 

 the ventral surface of that muscle. As this takes place the fibre-bundles become applied to each 

 surface of a flat tendon, which, after emerging from the muscle, is six to eight cm. long and three 

 to five cm. broad, and is inserted into the ventral side of the crest of the lesser tubercle of the 

 humerus and into the depth of the intertubercular (bicipital) groove immediately ventral to 

 the tendon of the teres major. With this it is more or less closely bound, although between 

 the tendons there lies a serous bursa. Some of the fasciculi of the tendon extend to the crest 

 of the greater tubercle. Frequently a tendon slip passes from the inferior margin of the ten- 

 don to the tendon on the posterior surface of the long head of the triceps or into the brachial 

 fascia (see latissimo-condyloideus, p. 379). 



Like the teres major, with wliich it is closely associated, the latissimus dorsi muscle under- 

 goes a torsion between its origin and its insertion, so that the dorsal surface of the nmscle is 

 continued into the ventral surface of the tendon and the most cranially situated of the fibre- 

 bundles are most distally attached to the humerus, and vice versa. The muscle either directly 

 or through its fascial extension is often adherent to the inferior angle of the scapula. 



Nerve-supply. — From the dorsal thoracic (long subscapular) nerve (from the sixth, seventh 

 and eighth cervical nerves). This nerve, which may arise in conjunction with the axillary nerve, 

 pas.ses to the deep surface of the mu.scle in the lower part of the axilla, and here gives rise to 

 rami which diverge as the muscle expands toward its tendons of origin. Though soon embedded 

 in the muscle substance, two main branches may he followed for a considerable distance near 

 the deep surface of the muscle. One usually extends near the lateral, the other near the supe- 

 rior, border of the muscle, and from these large rami pass into the intervening region. Branches 

 of the dorsal thoracic artery and vein accompany the nerve. 



Action.- — With the trunk fixed, the latissimus dorsi draws the raised arm down and back- 

 ward and rotates it mcdialward (.swimming movemont). When the arm is hanging by the side, 

 the action of the muscle is on the Hcay)ula. The upper third of the muscle (h-aws the scapula 

 toward the spine, the inferior two-tliirds depress the shoulder. When the humerus is fixed, 

 the latissimus serves to lift the trunk and pelvis forward, as in climbing. It also aids in forced 

 inspiration through its costal attachments. 



