176 HANDBOOK OF ANATOMY 



the whole length of the cord, and in transverse section re- 

 sembles in shape the letter H, the four ends of which are called 

 horns. 



There are thirty-one pairs of spinal nerves, which are attached 

 by two roots each to the lateral aspects of the cord opposite the 

 anterior and posterior horns of grey matter on each side respec- 

 tively. These nerves come through the intervertebral foramina,, 

 and are named according to the vertebrae below which they 

 emerge, except in the case of the cervical ones. The first 

 nerve comes out between the occipital bone and the atlas, so^ 

 that there are eight pairs of cervical nerves, twelve pairs of 

 dorsal nerves, five pairs of lumbar nerves, five pairs of sacral 

 nerves, and one pair of coccygeal nerves. As the cord itself 

 ends in the upper lumbar region, the lower nerves lie in the ver- 

 tebral canal, forming what is known as the cauda equina, and 

 emerging in order through the intervertebral foramina. 



The spinal nerves are attached to the cord by two roots 

 anterior and posterior which join together inside the vertebral 

 canal so that a mixed nerve i.e., formed by the junction of the 

 two roots emerges from the intervertebral foramen. It im- 

 mediately divides into anterior and posterior divisions. 



The posterior divisions supply the skin at the back of the 

 head, neck, shoulder, buttock, and trunk, and the longitudinal 

 muscles of the back. Each divides into two parts an internal 

 and external trunk. In the upper half of the body the in- 

 ternal trunks are cutaneous, the external ones muscular; in 

 the lower half of the body the reverse is the case. The pos- 

 terior divisions of the first and second cervical nerves vary a 

 little. 



First cervical nerve (suboccipital) does not divide into internal 

 and external'trunks, and gives off no cutaneous branches. Mus- 

 cular branches to complexus, rectus capitis posticus major and 

 minor, obliquus superior and inferior. 



Second cervical nerve (great occipital) supplies the skin at the 

 back of the head as far as the vertex, and communicates with 

 the other cutaneous nerves of that region. 



Muscular branches to complexus, obliquus inferior, and other 

 muscles on the back of the neck. 



