DISSECTION OF THE BACK 55 



turning forwards over the shoulder, are inserted into the 

 posterior border of the lateral third of the clavicle (Fig. 28); 

 the lower cervical and upper thoracic fibres pass more or less 

 transversely to gain an insertion into the medial border of 

 the acromion and the upper margin of the posterior border 

 of the spine of the scapula ; while the lower thoracic fibres are 

 directed upwards and, at the vertebral border of the scapula, 

 end in a flat, triangular tendon, which plays over the smooth 

 surface at the apex of the scapular spine, and is inserted 

 into a rough tubercle on the spine immediately beyond that 

 surface (Fig. 38, p. 83). To facilitate the movement of the 

 tendon upon the bone a small bursa mucosa is interposed 

 between them. 



The trapezius is supplied by the accessory nerve and by 

 twigs from the third and fourth cervical nerves. It is an 

 elevator and depressor of the shoulder ; and a rotator and 

 adductor of the scapula. 



Dissection. The latissimus dorsi is now to be dissected. It 

 is a difficult muscle to clean, not only on account of the varying 

 direction of its fibres, but also because its upper part is generally 

 very thin, and its upper border ill-defined. Near the spines of 

 the vertebrae its upper portion is overlapped by the trapezius, but 

 in its greater part the muscle is subcutaneous. Both the super- 

 ficial and the deep fascia should be raised at the same time from 

 its surface, and its fibres may be stretched by raising the arm 

 and folding it under the neck. 



On the right side cut through the fascia along the upper border 

 of the muscle from the point where that margin disappears under 

 the trapezius to the posterior fold of the axilla, and work down- 

 wards. Define carefully the attachment to the superficial layer 

 of the lumbo-dorsal fascia. Clean that fascia, but do not injure it ; 

 it is the property of the dissector of the head and neck. Next 

 define the attachment of the lower end of the muscle to the crest 

 of the ilium ; and when the lower part of the lateral border of the 

 muscle is reached, clean the slips of the latissimus which are 

 attached to the lowest three or four ribs, and clean also the slips 

 of the external oblique muscle of the abdomen which inter- 

 digitate with the costal slips of the latissimus dorsi. 



On the left side cut through the fascia from the crest of the 

 ilium to the posterior fold of the axilla, and work medially and 

 upwards ; and when the main part of the muscle has been cleaned, 

 return to the lower part of the lateral border and display the 

 costal attachments of the muscle and the interdigitations of the 

 external oblique. 



After the costal slips of the muscle have been defined, evert 

 the upper margin of the muscle, as it crosses the inferior angle of 

 the scapula, to display the slip of fibres which springs from that 

 angle and joins the deep aspect of the upper border of the muscle ; 

 it is apt to be mistaken for a piece of the teres major upon which 

 it lies, 

 i 4c 



