164 THE HUMAN BODY. 



part to the skin and muscles on the back, the second form 

 a series of plexuses from which the nerves for the sides and 

 ventral region of the neck and trunk and for the limbs arise; 

 the communicating branches go to neighboring sympathetic 

 ganglia. 



The various spinal nerves are named from the portions of 

 the vertebral column through the intervertebral foramina of 

 which they pass out; and as a general rule each nerve is named 

 from the vertebra in front of it. For example the nerve pass- 

 ing out between the fifth and sixth thoracic vertebrae is the 

 " fifth thoracic" nerve, and that between the last thoracic and 

 first lumbar vertebrae, the " twelfth thoracic." In the cervi- 

 cal region, however, this rule is not adhered to. The nerve 

 passing out between the occipital bone and the atlas is called 

 the "first cervical" nerve, that between the atlas and axis the 

 second, and so on; that between seventh cervical and first 

 thoracic vertebrae being the "eighth cervical" nerve. The 

 thirty-one pairs of spinal nerves are then thus distributed: 8 

 cervical, 12 thoracic, 5 lumbar, 5 sacral, and 1 coccygeal; the 

 latter passing out between the sacrum and coccyx. Since the 

 spinal cord ends opposite the upper lumbar vertebrae while 

 the sacral and coccygeal nerves pass out from the neural canal 

 much farther back, it is clear that the roots of those nerves, 

 on their way to unite in the foramina of exit and form nerve- 

 trunks, must run obliquely backwards in the spinal canal for 

 a considerable distance. One finds in fact the neural canal 

 in the lumbar and sacral regions, behind the point where the 

 spinal cord has tapered off to form the filum terminale, oc- 

 cupied chiefly by a great bunch of nerve-roots forming the 

 so-called "horse's tail" or cauda equina. 



Distribution of the Spinal Nerves. It would be out of 

 place here to go into detail as to the exact portions of the 

 Body supplied by each spinal nerve, but the following general 

 statements may be made. The anterior primary branches of 

 the first four cervical nerves form on each side the cervical 

 plexus (Fig. 72) from which branches are supplied to the 

 muscles and integument of the neck: also to the outer ear 

 and the back part of the scalp. The anterior primary 

 branches of the remaining cervical nerves and the first dorsal 

 form the Irachial plexus, from which the upper limb is 

 supplied. The roots of the trunks which form this plexus 

 arise from the cervical enlargement of the spinal cord. 



