656 SPINAL NERVES. 



The two lower posterior sacral nerves are situated below the Multifidus Spinse. 

 They are of small size, and join with each other, and with the coccygeal nerve, 

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

 the integument over the coccyx. 



The coccygeal nerve divides into its anterior and posterior branch in the spinal 

 canal. The posterior branch is the smaller. It receives, as already mentioned, 

 a communicating branch from the last sacral, and is lost in the fibrous struc- 

 ture 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 sym- 

 pathetic, 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 vis- 

 ceral 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 sympathetic to form the hypogastric plexus. The muscular branches are 

 distributed to the Levator Ani, Coccygeus, and Sphincter Ani. Cutaneous fila- 

 ments 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 

 integument over the back part and side of the coccyx. This nerve commu- 

 nicates above with the fourth sacral, 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 be- 

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



SACRAL PLEXUS. 



The Sacral Plexus is formed by the lumbo-sacral, the anterior branches of 

 the three upper sacral nerves, and part of that of the fourth. 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 Ob- 

 turator Internus 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- 



