THE SACRAL PLEXUS. 859 



ani, Coccygeus, and Sphincter ani. The branch to the Sphincter ani pierces the 

 Levator ani, so as to reach the ischio-rectal fossa, where it is found lying in front 

 of the coccyx. Cutaneous filaments arise from the latter branch, which supply 

 the integument between the anus and coccyx. Another cutaneous branch is fre- 

 quentlv given off from this nerve, though sometimes from the pudic (Schwalbe). 

 It perforates the great sacro-sciatic ligament, and, winding round the lower bor- 

 der of the Gluteus maximus, supplies the skin over the lower and inner part of 

 this muscle. 



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

 canal, curves forward through the fifth sacral foramen, formed between the lower 

 part of the sacrum and the transverse process of the first piece of the coccyx. It 

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

 tip of the coccyx, where it again perforates the muscle, to be distributed to the 

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

 above with the fourth sacral and below with the coccygeal nerve, and supplies the 

 Coccygeus muscle. 



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

 at the termination of the sacral canal ; it passes downward behind the rudiment- 

 ary transverse process of the first piece of the coccyx, and curves forward through 

 the notch between the first and second pieces, piercing the Coccygeus muscle, and 

 descending on its anterior surface to near the tip of the coccyx, where it again 

 pierces the muscle, to be distributed to the integument over the back part and side 

 of the coccyx. It is joined by a branch from the fifth anterior sacral as it 

 descends on the surface of the Coccygeus muscle. 



The Sacral Plexus (Fig. 508). 



The sacral plexus is formed by the lumbo-sacral cord, the anterior divisions of 

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

 in different directions ; the upper ones obliquely downward and outward, the 

 lower ones nearly horizontally. 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 special method of the formation of the plexus is as follows : The lumbo- 

 sacral cord, first, second and larger part of the third sacral nerves unite to form a 

 large upper cord or band. The smaller part of the third, together with the branch 

 of the fourth nerve, already mentioned as going to the sacral plexus, unite to 

 form a smaller, lower, cord or band. The larger is continued into the great sciatic 

 nerve ; the smaller is continuous with the pudic nerve. The remaining branches 

 of the plexus are derived separately or by more or less intercommunication from 

 the sacral nerves before the latter form the two principal cords just mentioned. 



The branches of the sacral plexus are 



Muscular. Perforating cutaneous. 



Superior gluteal. Pudic. 



Inferior gluteal. Small sciatic. 



Great sciatic. 



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

 Gemelli, and the Quadratus femoris. The branches to the Pyriformis arise from 

 the back of the first and second sacral nerves before they enter the plexus ; the 

 branch to the Obturator internus arises from the lumbo-sacral and first two sacral 

 nerves : it passes out of the pelvis through the great sacro-sciatic foramen, crosses 

 the spine of the ischium, and re-enters the pelvis through the lesser sacro-sciatic 

 foramen to the inner surface of the Obturator internus ; the branch to the 

 Gemellus superior arises in common with the nerve to the Obturator internus : it 



