THE DISSECTION OF THE BACK 175 



the ilio-costalis on the other. The upper six or seven of these 

 nerves are exhausted in the supply of the middle and lateral 

 columns of the sacrospinalis ; the lower five or six, however, 

 are considerably larger, and contain both motor and sensory 

 fibres. After giving up their motor fibres to the muscles, 

 they become superficial, by piercing the serratus posterior in- 

 ferior and the latissimus dorsi, in a line with the angles of 

 the ribs. The cutaneous distribution of these nerves has 

 already been examined by the dissector of the upper limb. 



The medial divisions also are distributed differently in the 

 upper and lower portions of the dorsal region. The lower 

 five or six are very small, and end in the multifidus 

 muscle. The upper six or seven pass medially between the 

 multifidus and semispinalis, and after supplying the muscles 

 between which they are situated, they become superficial. In 

 passing towards the surface they pierce the splenius, rhom- 

 boids, and trapezius muscles, and thus gain the superficial 

 fascia, where they have been dissected already. 



Lumbar Region. The medial divisions of the posterior 

 branches of the five lumbar nerves are small, and, like the 

 corresponding twigs in the lower dorsal region, they have 

 a purely muscular distribution. They end in the multifidus. 



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 upper three 

 of these nerves are of large size, and 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 lowest lateral 

 division 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 

 to these, however, minute twigs may be discovered, in a well- 

 injected subject, passing posteriorly from the vertebral artery 

 in the intervals between the transverse processes, and also in 

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

 anastomose with the other arteries in this region. 



In the dorsal region the posterior branches of the aortic inter- 



