518 DISSECTION OF THE PELVIS. 



vious dissections ; but if any of it remains the student may trace the 

 offsets distributed from it, and its communicating brandies with the spinal 

 nerves. 



SACRAL SPINAL NERVES (fig. 177). The anterior primary branches 

 of the sacral nerves are five in number, and decrease suddenly in size 

 from above downwards. Issuing by the apertures in the front of the 

 sacrum (the fifth nerve excepted) they receive short filaments of commu- 

 nication from the gangliated cord of the sympathetic. The first three 

 nerves and part of the fourth enter the sacral plexus, but the fifth ends on 

 the back of the coccyx. 



The coccygeal nerve, and the peculiarities of the fourth and fifth sacral, 

 will be noticed before the plexus is described. 



The fourth nerve (4 S) divides into two parts : one communicates 

 with the sacral plexus ; the other joins the fifth nerve and distributes the 

 following branches to the viscera and the surrounding muscles : 



The visceral branches (3) supply the bladder and the vagina, and com- 

 municate with the sympathetic nerve to form the pelvic plexus. Some- 

 times these branches come from the third sacral nerve. 



The muscular branches are three in number. One (2) rather long 

 branch enters the levator ani on the visceral aspect ; another (4) supplies 

 the coccygeus ; and the third (5) reaches the perinreum by piercing the 

 levator ani muscle (p. 391). 



The fifth nerve (5 S) comes forwards between the sacrum and coccyx, 

 and receives the communicating branch from the fourth nerve ; it is then 

 directed downwards in front of the coccygeus, where it is joined by the 

 coccygeal nerve, and perforates that muscle, the sacro-sciatic ligament, 

 and the gluteus maximus, to end on the posterior surface of the coccyx. 



The coccygeal nerve (1 c) after issuing by the lower aperture of the 

 spinal canal, appears through the coccygeus muscle, and joins the fifth 

 sacral nerve as above stated. 



SACRAL PLEXUS (fig. 177). This plexus is formed by the lumbo-sacral 

 cord, the first three sacral nerves, and part of the fourth sacral. It is 

 situate on the pyriformis muscle, beneath the sciatic and pudic branches 

 of the internal iliac artery ; and the nerves entering it converge towards 

 the large sacro-sciatic notch, to unite in a flat band. From that spot the 

 plexus becomes gradually smaller towards the outer end ; and, leaving the 

 pelvis below the pyriformis, terminates in branches for the limb at the 

 lower border of that muscle. 



Branches. Most of the branches arise outside the pelvis, and are dis- 

 tributed to the back of the lower limb ; for, only two internal pelvic 

 muscles (pyriformis and obturator internus) receive nerves from it. 



The nerve of the obturator internus springs from the part of the plexus 

 formed by the union of the lumbo-sacral with the first sacral nerve ; it 

 leaves the pelvis with the pudic artery, and winding over the ischial spine 

 of the hip bone and through the small sacro-sciatic notch, enters the 

 muscle on the perinatal surface. 



The nerves of the pyriformis are commonly two in number, and arise 

 from separate parts of the plexus : they enter the muscle at its visceral 

 aspect. 



The pudic nerve, like the artery of the same name, supplies the parts in 

 the perineum, and the genital organs. The nerve arises in the lower part 

 of the plexus, and courses over the small sacro-sciatic ligament, to accom- 

 pany its artery through the small sacro-sciatic notch. 



