THE SACRAL NERVES. 



857 



THE SACRAL AND COCCYGEAL NERVES. 



The sacral nerves are five in number on each side. The four upper ones pass 

 from the sacral canal through the sacral foramina ; the fifth through the foramen 

 between the sacrum and coccyx. 



The roots of origin of the upper sacral (and lower lumbar) nerves are the 

 largest of all the spinal nerves, whilst those of the lowest sacral and coccygeal 

 nerve are the smallest. 



The roots of these nerves are of very considerable length, being longer than 

 those of any of the other spinal nerves, on account of the spinal cord not extending 

 beyond the first lumbar vertebra. From their great length and the appearance 

 they present in connection with the spinal cord the roots of origin of these nerves 

 are called collectively the cauda equina. Each sacral and coccygeal nerve divides 

 into two divisions, posterior and anterior. 



The posterior divisions of the sacral nerves (Fig. 507) are small, diminish in 





FIG. 507. The posterior sacral nerves. 



size from above downward, and emerge, except the last, from -the sacral canal 

 by the posterior sacral foramina. 



The three upper ones are covered, at their exit from the sacral canal, by the 

 Multifidus spinse, and divide into internal and external branches. 



The internal branches are small, and supply the Multifidus spinse. 



The external branches join with one another and with the last lumbar and 

 fourth sacral nerves by means of communicating loops. These branches pass out- 

 ward to the outer surface of the great sacro-sciatic ligament, where they form a 

 second series of loops beneath the Gluteus maximus. Cutaneous branches from 

 this second series of loops, usually three in number, pierce the Gluteus maximus : 

 one near the posterior inferior spine of the ilium ; another opposite the end of the 

 sacrum ; and the third midway between the other two. They supply the integu- 

 ment over the posterior part of the gluteal region (nervi clunium medii). 



