738 THE NEK VOUS SYSTEM. 



from the back of the second and third sacral nerves (Fig. 631, p. 736). At 

 its origin it is associated with the lower roots of the posterior cutaneous nerve 

 of the thigh. Passing dis tally it pierces the sacro - tuberous ligament, 

 along with the coccygeal branch of the inferior gluteal artery ; and after winding 

 round the lower border of the glutseus maximus muscle, or in some cases piercing 

 its lower fibres, it becomes cutaneous a little distance from the coccyx, and supplies 

 the skin over the lower part of the buttock and the medial part of the fold of the 

 nates. 



The perforating cutaneous nerve is not always present. In a minority of cases it is associated 

 at its origin with the pudendal nerve. When absent as a separate nerve, its place is taken by (1) 

 gluteal branches of the posterior cutaneous nerve of the thigh, or (2) a branch from the pudendal 

 nerve, or (3) a small nerve (n. perforans coccygeus major, Eisler), arising separately from the 

 posterior part of the third and fourth sacral nerves. 



Muscular Branches. Between the third and fourth sacral nerves (occasion- 

 ally reinforced by the second, Eisler) a plexiform loop is formed, from which 

 muscular nerves are given off to the levator ani (supplying the muscle on its pelvic 

 surface), coccygeus, and external sphincter. The nerve to the external sphincter 

 (perineal branch of fourth sacral) pierces the sacro-tuberous ligament and the 

 coccygeus muscle, to which it gives offsets, and appears in the ischio-rectal fossa 

 between the glutseus maximus and the external sphincter. Besides supplying the 

 posterior fibres of the external sphincter, it distributes cutaneous offsets to the skin 

 of the ischio-rectal fossa and the fold of the nates behind the anus. This nerve, 

 in some instances, replaces the perforating cutaneous nerve. 



Nn. Anococcygei (Ano-coccygeal Nerve). By the union of the remaining 

 part of the fourth with the fifth sacral and coccygeal nerves, the so-called 

 plexus coccygeus (coccygeal plexus) is formed. A fine descending branch of the 

 fourth sacral nerve passes over or through the sacro-tuberous ligament, to join the 

 fifth sacral nerve. This fifth sacral nerve, joined by the descending branch of the 

 fourth, descends alongside the coccyx and is again joined by the coccygeal nerve, 

 so that a plexiform cord, the ano-coccygeal nerve results, homologous with the inferior 

 caudal trunk of tailed animals. Fine twigs arise from it, which pierce the sacro- 

 tuberous ligament and supply the skin in the neighbourhood of the coccyx, medial 

 to the branches of the perforating cutaneous nerve and behind the anus. 



NERVUS PUDENDUS. 



The pudendal nerve (O.T. pudic) is the principal nerve for the supply of the 

 perineum. It arises in the pelvis usually by three roots from the second, third, and 

 fourth sacral nerves (Fig. 631, p. 736). (Frequently one of its branches, the inferior 

 hsemorrhoidal nerve, arises independently from the third and fourth sacral nerves.} 

 The nerve passes to the buttock through the greater sciatic foramen, below the 

 sciatic nerve, and lies on the sacro-spinous ligament, or the spine of the ischium 

 medial to the internal pudendal artery. It enters the perineum along with thf 

 artery through the lesser sciatic foramen. In the perineum it is deeply placed ir 

 the lateral wall of the ischio-rectal fossa, enclosed in a special sheath derivec 

 from the parietal pelvic fascia covering the medial surface of the obturator in 

 ternus muscle. At the anterior limit of the ischio-rectal fossa, the nerve approache 

 the surface and divides at the base of the urogenital diaphragm into it 

 terminal branches, the perineal nerve and the dorsal nerve of the penis. 



The branches of the nerve are essentially the same in the two sexes. As ; 

 rule no branches are given off till it enters the perineum, but sometimes th 

 inferior hsemorrhoidal nerve has an independent origin from the plexus, rnerel 

 accompanying the pudendal nerve in the first part of its course ; and in exceptions 

 cases the perforating cutaneous nerve of the buttock is a branch of the pudenda 

 nerve. 



Nn. Haemorrhoidales Inferiores. The inferior hsemorrhoidal nerve aris< 

 from the pudendal nerve under cover of the glutaeus maximus, at the posterior part < 

 the ischio-rectal fossa. In cases in which it has an independent origin from the plexu 

 it arises from the third and fourth sacral nerves. It crosses the ischio-rectal fos^ 



