SACRAL PLEXUS. 585 



so as to form loops on the back of the sacrum, filaments from which supply the 

 gumeut over the coccyx. 



The posterior branch of the coccygeal nerve is small. It separates from the 

 anterior in the sacral canal, and receives, as already mentioned, a communicating 

 branch from the last sacral. It is lost in the fibrous structure on the back of the 

 coccyx. 



The anterior sacral nerves diminish in size from above downwards. The 

 four upper ones emerge from the anterior sacral foramina ; the anterior branch of 

 the fifth, together with the coccygeal nerve, between the sacrum and the coccyx. 

 All the anterior sacral nerves communicate with the sacral ganglia of the sympa- 

 thetic, at their exit from the sacral foramina. The first nerve, of large size, unites 

 with the lumbo-sacral nerve. The second equals in size the preceding, with which 

 it joins. The third, about one-fourth the size of the second, unites with the 

 preceding nerves, to form the sacral plexus. 



. The fourth anterior sacral nerve sends a branch to join the sacral plexus. 

 The remaining portion of the nerve divides into visceral and muscular branches ; 

 and a communicating filament descends to join the fifth sacral nerve. The visceral 

 branches are distributed to the viscera of the pelvis, communicating with the 

 sympathetic nerve. These branches ascend upon the rectum and bladder ; in the 

 female, upon the vagina and bladder, communicating with branches of the sympa- 

 thetic to form the hypogastric plexus. The muscular branches are distributed to 

 the Levator ani, Coccygeus, and Sphincter ani. Cutaneous filaments arise from 

 the latter branch, which supply the integument between the anus and coccyx. 



The fifth anterior sacral nerve, after passing from the lower end of the sacral 

 canal, pierces the Coccygeus muscle, and descends upon its anterior surface to the 

 tip of the coccyx, where it perforates that muscle, to be distributed to the integu- 

 ment over the back- part and side of the coccyx. This nerve communicates above 

 with the fourth, and below with the coccygeal nerve, and supplies the Coccygeus 

 muscle. 



The anterior branch of the coccygeal nerve is a delicate filament which escapes 

 at the termination of the sacral canal. It pierces the sacro-sciatic ligament and 

 Coccygeus muscle, is joined by a branch from the fifth anterior sacral, and 

 becomes lost in the integument at the back part and side of the coccyx. 



SACRAL PLEXUS. 



The sacral plexus is formed by the lurnbo-sacral, the anterior branches of the 

 three upper, and part of the fourth sacral nerves. These nerves proceed in 

 different directions ; the upper ones obliquely outwards, the lower one nearly 

 horizontally, and they all unite into a single, broad, flat cord. The sacral plexus 

 is triangular in form, its base corresponding with the exit of the nerves from the 

 sacrum, its apex with the lower part of the great sacro-sciatic foramen. It rests 

 upon the anterior surface of the Pyriformis, and is covered in front by the pelvic 

 fascia, which separates it from the sciatic and pudic branches of the internal iliac 

 artery, and from the viscera of the pelvis. 



The branches of the sacral plexus are : 



Muscular. Pudic. 



Superior gluteal. Small sciatic. 



Great sciatic. 



The muscular branches supply the Pyriformis, Obturator internus, the two 

 Gemelli, and the Quadratus femoris. The branch to the Pyriformis arises either 

 from the plexus, or from the upper sacral nerves ; the branch to the Obturator in- 

 ternus arises at the junction of the lumbo-sacral and first sacral nerves ; it crosses 

 behind the spine of the ischium, and passes through the lesser sacro-sciatic foramen 

 to the inner surface of the Obturator internus ; the branch to the Gemellus superior 

 arises from the lower part of the plexus, near the pudio nerve ; the small branch 



