790 THE NERVOUS SYSTEM. 



bar. The second, equal in size to the preceding, and the third, about one-fourth 

 the size of the second, unite with this trunk, and form, with a small fasciculus 

 from the fourth, the sacral plexus, a visceral branch being given off from the 

 third nerve to the bladder. 



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, and 

 in the female upon the vagina, communicating with branches of the sympathetic 

 from the pelvic plexus. The muscular branches are distributed to the Levator 

 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- 

 quently 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. 421). 



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 pro- 

 ceed in different directions : the upper ones obliquely downward and outward, the 

 lower ones nearly horizontally, and they all unite into two cords : an upper and 

 larger, which is formed by the lumbo-sacral cord with the first, second, and the 

 greater part of the third sacral nerves ; and a lower and smaller, formed by the 

 remainder of the third, Avith a portion of the fourth sacral nerve. The upper 

 cord is prolonged into the great sciatic nerve and the lower into the pudic. Fre- 

 quently a small filament is given off from the second sacral nerve to join the 

 lower 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 saci'al plexus are : 



