THE DISSECTION OF THE BACK 69 



with the descending branch of the occipital. Both vessels 

 anastomose with twigs from the vertebral artery. 



The arteria profunda cervicis is accompanied by a large 

 vein the vena profunda cervicis. That vessel begins in the 

 sub-occipital plexus, and it ends in the vertebral vein close to 

 its termination. It reaches its termination by turning forwards 

 under the transverse process of the last cervical vertebra. 



Posterior Rami of the Spinal Nerves. The nerves of the 

 back must now be examined. They are the posterior rami 

 of the spinal nerves. With -four exceptions (viz., the first 

 cervical, fourth and fifth sacral, and the coccygeal nerves), 

 each posterior ramus will be found to divide into a lateral 

 and a medial division. 



Examine the nerves successively in the cervical, thoracic, and lumbar 

 regions. It is well, however, to defer the dissection of the sacral and 

 coccygeal nerves until the multifidus muscle has been studied. 



Cervical Region. In the neck the posterior rami 

 of the spinal nerves are eight in number. The posterior 

 ramus of the sub-occipital or first spinal nerve fails to divide into 

 a medial and a lateral division. It lies deeply in the sub- 

 occipital triangle, and will be examined when that space is 

 dissected. 



The posterior ramus of the second cervical nerve is very 

 large. It appears between the vertebral arches of the first 

 and the second cervical vertebrae. The posterior rami of the 

 succeeding six cerincal nerves arise from the corresponding 

 spinal nerve-trunks in the intervertebral foramina, but they 

 turn dorsally, on the medial sides of the posterior inter- 

 transverse muscles, and appear in the intervals between the 

 transverse processes. 



The lateral divisions are of small size, and are entirely 

 devoted to the supply of adjacent muscles. 



The medial divisions are not all distributed alike, nor 

 indeed do they present the same relations. Those from the 

 second^ third, fourth, and fifth nerves run medially towards the 

 spinous processes, superficial to the semispinalis cervicis 

 muscle, and under cover of the semispinalis capitis. When 

 close to the median plane they turn backwards, pierce the 

 semispinalis capitis, splenius, and trapezius muscles, and 

 become superficial. In their course to the surface they give 

 numerous twigs to the neighbouring muscles. 



The medial division of the second nerve is remarkable for 



