956 THE NERVOUS SYSTEM 



TABLE SHOWING RELATIONS OF MUSCLES OF LOWER EXTREMITY TO NERVES OP 



LUMBAR AND SACRAL PLEXUSES (Continued) 

 NERVES CONTRIBUTING. MUSCLES. NERVES. 



Ext. hall. long. Deep peroneal 



" digit. " 

 digit, brev. 



4, 5 L., and 1 S. 



TibiaHs anterior 



Peroneus tertius 



" longus Superficial peroneal 



" brevis " peroneal 



PUDENDAL PLEXUS 



The pudendal plexus, like the parts of the lumbo-sacral plexus -already described, 

 varies in its formation. The accompanying tables show the extreme range of varia- 

 tion and the common method of formation of the large nerve of this plexus in each 

 of the three classes. 



COMPOSITION OF THE NERVES OF THE PUDENDAL PLEXUS 



RANGE OF VARIATION 



NERVE PROXIMAL. ORDINARY. DISTAL. 



Pudic nerve 1,2,3,4,58. 1,2,3,48. 2,3,4,58. 



COMMON COMPOSITION 



NERVE PROXIMAL. ORDINARY. DISTAL. 



Pudic nerve 2,38. 2, 3, 4S. 3,48. 



The pudendal plexus is commonly formed by part of the anterior divisions of the 

 second, third, and fourth sacral nerves. It lies in the lower part of the back of the 

 pelvis, and gives off visceral, muscular, and terminal branches. 



Visceral branches (pelvic splanchmcs) arise from the third and fourth sacral 

 nerves especially, and communicate with branches of the sympathetic plexus. 

 They are distributed either directly or through this plexus to the pelvic viscera (figs. 

 688, 692, 693). The middle haemorrhoidal nerves pass to the rectum, the inferior 

 vesical nerves to the bladder, and, in the female, the vaginal nerves to the vagina 

 (see SYMPATHETIC SYSTEM). 



Muscular branches are given by the fourth sacral nerve to the coccygeus, levator 

 ani, and sphincter ani externus (figs. 688, 692). 



The nerves to the two former muscles pass into the pelvic surfaces of the muscles, 

 but that to the last named muscle, called the perineal branch, passes backwards be- 

 tween the levator ani and the coccygeus, or through the posterior fibres of the latter 

 muscle, into the posterior part of the ischio-rectal fossa, and, in addition to. sup- 

 plying the sphincter ani, it gives cutaneous filaments to the skin between the anus 

 and the coccyx. 



Terminal branches. The pudic nerve (n. pudendus) rises usually from the 

 anterior primary divisions of the second, third, and fourth sacral nerves (fig. 692). 

 It emerges from the pelvis below the piriformis, crosses the spine of the ischium, 

 lying to the inner side of the internal pudic artery (fig. 693), and accompanies the 

 artery, through the small sciatic foramen, into Alcock's canal in the obturator fascia 

 on the outer wall of the ischio-rectal fossa, where it terminates by dividing into three 

 branches, the inferior haemorrhoidal, the perineal, and the dorsal nerve of the penis. 



The inferior hsemorrhoidal nerves frequently arise independently from the 

 third and fourth sacral nerves, pierce the inner wall of Alcock's canal, and pass for- 

 wards and inwards through the ischio-rectal fat to supply the sphincter ani externus 

 and adjacent skin. They anastomose with branches of the perineal nerve. 



The perineal nerve runs forwards for a short distance in Alcock's canal and 

 divides into a deep and a superficial branch. The deep branch breaks up into fila- 

 ments, one or two of which pierce the inner wall of the canal and pass inwards to 

 the anterior fibres of the sphincter and levator ani. The remaining part of the nerve 

 pierces the base of the uro-genital trigone (triangular ligament) , and enters the super- 

 ficial pouch of the urethral triangle, where it is distributed to the bulb of the urethra, 



