THE DISSECTION OF THE BACK 71 



The lateral divisions sink into the substance of the sacro- 

 spinalis, and are concerned in the supply of that muscle, 

 and also of the lumbar intertransverse muscles. The lateral 

 divisions of the upper three lumbar nerves are of large size, 

 they become cutaneous by piercing the superficial lamella of 

 the lumbo- dorsal fascia. They have already been traced by 

 the dissector of the lower limb to the skin of the gluteal 

 region. The lateral division of the fifth communicates with 

 the corresponding branch of the first sacral nerve. 



Blood Vessels of the Back. In the cervical region the 

 dissector has already noticed the arteria profunda cervicis, 

 and the descending branch of the second part of the occipital 

 artery. Deep in the sub-occipital region he will subsequently 

 meet with a small portion of the vertebral artery. In addition, 

 however, minute twigs from the vertebral artery may be 

 discovered, in a well-injected subject, passing backwards in 

 the intervals between the transverse processes, and also in 

 the sub -occipital space. They supply the muscles, and 

 anastomose with the other arteries in that region. 



In the thoracic region the posterior branches of the aortic 

 intercostal arteries and superior intercostal artery make 

 their appearance between the transverse processes. Each of 

 them passes dorsally in the interval between the body of a 

 vertebra and the costo-transverse ligament. It is associated 

 with the corresponding posterior ramus of a spinal nerve, and 

 is discributed, with the nerve, to the muscles and integument 

 of the back. 



In the lumbar region similar branches are derived from the 

 lumbar arteries. They are distributed in the same manner. 



In both thoracic and lumbar regions, before reaching 

 the back, the vessels under discussion furnish small spinal 

 branches which enter the vertebral canal through the inter- 

 vertebral foramina. These will be traced at a later period. 



The veins accompanying the dorsal branches of the lumbar 

 and intercostal arteries pour their blood into the lumbar 

 and intercostal veins. They are of large size, being joined 

 by tributaries from the posterior vertebral venous plexus, and 

 also by others from within the vertebral canal. 



Dissection. The remainder of the third layer of spinal 

 muscles must now be dissected. They are the semispinalis dorsi 

 and semispinalis cervicis. The semispinalis cervicis is already 

 exposed ; but to display the semispinalis dorsi it is necessary to 

 remove the spinalis dorsi muscle, 

 in 5 6 



