574 HUMAN ANATOMY. 



ments. It is directed downward and laterally and is inserted into the lower two- 

 thirds of the vertebral border of the scapula. 



Nerve-Supply. — By the dorsal scapular nerve from the fifth cervical nerve. 



Action. — To draw the scapula upward and medially, at the same time rotating 

 it so that the lateral angle is moved downward. 



Variations of the Rhomboidei. — The rhomboidei are sometimes entirely wanting, and 

 the origins of both muscles may be e.xtended beyond the usual limits. 



The occipito-scapularis is a muscle occasionally present which is intimately associated in 

 its derivation with the rhomboids. It arises from the inner part of the superior nuchal line and 

 passes downward between the trapezius and splenius to join the rhomboideus minor, inserting 

 with it into the vertebral border of the scapula. 



5. Latissimus Dorsi (Fig. 559). 



Attachments. — The latissimus dorsi is a large triangular muscle which arises 

 from the spinous processes of the last six thoracic vertebrae and the intervening 

 interspinous ligaments beneath the origin of the trapezius, from the lumbo-dorsal 

 fascia, from the posterior portion of the crest of the ilium, and by fleshy digitations 

 from the outer surfaces of the lower three or four ribs. Its fibres pass upward and 

 laterally over the inferior angle of the scapula, from which an additional slip is usu- 

 ally added to the muscle. It then curves around the lower border of the teres 

 major and is inserted^ ventrally to that muscle, into the crest of the inner tuberosity 

 of the humerus. A mucous bursa {bursa vi. latissimi dorsi ) lies between the tendons 

 of insertion of the latissimus dorsi and teres major. 



Nerve-Supply. — By the long subscapular nerve from the seventh and eighth 

 cervical nerves. 



Action. — -To draw the humerus downward, backward, and inward, at the same 

 time rotating it inward, the action being that of the arm in swimming. If the 

 humerus be fi.xed, as in climbing, it draws the pelvis and lower portion of the trunk 

 upward and forward. 



Variations. — The latissimus dorsi, like the serratus anterior and pectorales, is a muscle 

 which has migrated extensively from the region of its first formation, the lower cer\ical region, 

 and this migration can be witnessed in the ontogeny of the muscle. Consequently variations 

 may be expected and do occur in the extent of the origin of the muscle, whose descent and 

 backward migration to the vertebral column may be interrupted at various stages. 



A great amount of variation of this nature is seen in its attachment to the crest of the ilium. 

 In some cases this attachment extends so far forward as to meet the posterior extremity of the 

 attachment of the external oblique of the abdomen, but usually this does not occur, and a tri- 

 angular interval, known as the triangle of Petit, occurs between the borders of the two muscles 

 and above the crest of the ilium. The floor of the triangle is formed by the internal obliquus 

 abdominis, and, since the abdominal wall is here thinner than elsewhere, the triangle may occa- 

 sionally be the seat of a lumbar hernia. 



Closely allied to the latissimus dorsi is a muscle, the ni. dorso-epitrochlearis, which occurs 

 in 18 or 20 per cent, of cases. It takes its origin from the body or tendon of insertion of the 

 latissimus and passes to the brachial fascia or to the medial epicondyle of the humerus. It has 

 been regarded as an aberrant portion of the pectoralis group of muscles, but its supply by the 

 musculo-spiral nerve places it among the post-axial muscles. 



The Axillary Fascia. — The axillary fascia is a firm sheet which extends 

 across from the lower border of the pectoralis major to that of the latissimus dorsi 

 and teres major, forming the floor of the axilla. Laterally it passes over into the 

 deep fascia of the arm, medially into the fascia covering the serratus magnus, and 

 near the border of the pectoralis major it has inserted into it the downward continu- 

 ation of the fascia which encloses the pectoralis minor (Fig. 556). It is pierced by 

 numerous lymphatic vessels, and along its medial edge is considerably thickened 

 to form a curved band, whose concavity is directed laterally, and which stretches 

 across between the tendons of the pectoralis major and the latissimus, forming 

 what is termed the axillary arch. Muscle-fibres are occasionally found in this arch 

 (page 571 )._ 



The axilla is a pyramidal space intervening between the upper part of the 

 brachium and the lateral wall of the thorax. Its apex is directed upward and the 



