THE SPINAL NERVES. 161 



atlas is called the "first cervical" nerve, that between the- 

 atlas and axis the second, and so on; that between seventh 

 cervical and first dorsal vertebrae being the "eighth cervi- 

 cal " nerve. The thirty-one pairs of spinal nerves are then 

 thus distributed: 8 cervical, 12 dorsal, 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, occupied 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. 65) from which branches are sup- 

 plied 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 brackial 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. 



From the fourth and fifth cervical nerves on each side, 

 small branches arise and unite to make the phrenic nerve 

 (4', Fig. 65) which runs down through the chest and ends 

 in the diaphragm. 



The anterior primary branches of the dorsal nerves, ex- 

 cept part of the first which enters the brachial plexus, form 

 no plexus, but each runs along the posterior border of a rib 

 and supplies branches to the chest-walls, and the lower ones 

 to those of the abdomen also. 



The anterior primary branches of the four anterior lum- 

 bar nerves are united by branches to form the lumbar 



