CHAPTER III. 



DISSECTION OF THE BACK AND THORAX. 



The dissection of the thorax should be begun at the same time as that 

 of the outer scapular region (see page 8). 



THE CHEST-WALL AND BACK. 



Directions. — The portion of skin remaining on the chest- wall and loins 

 should be removed, the operation being commenced at an incision 

 carried along the middle line, from the withers to the croup. The cut- 

 aneous nerves must then be sought, after which the other structure s 

 are to be taken up in the order of their description. 



Cutaneous Nerves. In the back these are derived from the dorsal 

 nerves. One set of branches appears close to the spinous processes ; 

 and another a few inches outwards, along the course of the longissimus 

 dorsi muscle. Both of these are derived from the superior primary- 

 branches of the dorsal nerves. Over the sides of the chest the cutaneous 

 nerves are derived from the perforating branches of the intercostal 

 nerves, which are dissected with the abdominal muscles (see page 288). 



In the loins the cutaneous nerves are derived from the superior 

 primary branches of the lumbar nerves, and the most posterior of them 

 are continued backwards to the skin over the gluteal region. 



The Panniculus Caenosus (Plate 38). This is an extensive muscle 

 adherent to the deep surface of the skin over a large part of the abdo- 

 men, thorax, and shoulder. It is fully described at page 287, which see. 



Directions. — The panniculus should now be entirely removed, begin- 

 ning at its upper border. 



The Latissimus Dorsi. This muscle is partly described at pages 

 9 and 14, in connection with the dissection of the fore limb. It 

 arises by a broad aponeurotic tendon from the series of vertebral spines, 

 beginning about the 4th dorsal, and extending back to the last lumbar. 

 This tendon is not well defined at its inferior border, where it is 

 adherent to the ribs, and blends with the oblique muscles of the 

 abdomen. Posteriorly the tendon becomes continuous with the gluteal 

 fascia. The tendon is succeeded by a thick muscular portion, which 

 contracts and passes to the inner side of the fore limb, where it becomes 

 inserted into the internal tubercle of the humerus. 



Action. — It is a flexor and an inward-rotator of the shoulder-joint. 



