402 



DISSECTION OF THE PELVIS. 



Fourth, 

 which gives 

 visceral, 



and muscu- 

 lar offsets. 



Fifth is be- 

 low aper- 

 tures in 

 sacrum ; 



ends on 

 coccyx. 



Coccygeal 

 nerve. 



Sacral 

 plexus ; 



situation ; 



how formed 

 ending ; 



and 

 branches : 



Great 

 sciatic. 



The coccygeal nerve and the peculiarities of the fourth and fifth 

 sacral will be noticed before the plexus is described. 



The FOURTH NERVE (fig. 149, 4 S) sends one branch upwards 

 to the sacral plexus, another downwards to join the fifth nerve, 

 . and distributes the following offsets to the viscera and the muscles 

 of the floor of the pelvis : 



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

 communicate with the sympathetic nerve to form the pelvic 

 plexus. Offsets are added to them from the third sacral nerve 

 (fig. 150, ). 



The muscular brandies are three in number. One rather long 

 branch (tig. 149, 2 ) enters the levator ani on the visceral aspect ; 

 another ( 4 ) supplies the coccygeus ; and the third (perineal) or 

 hsemorrhoidal branch ( 5 ) reaches the .perineum by piercing 

 the levator ani or coccygeus muscle, and supplies the external 

 sphincter. 



The FIFTH NERVE (5 $) 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 glutens 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 PLEXU8. 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 foramen. Here they are 

 united for the most part in a broad flat band, which, becoming 

 gradually narrower as it leaves the pelvis below the pyriformis, 

 forms the great sciatic nerve. A part of the third nerve, however, 

 and the branch of the fourth entering the plexus unite to form a 

 lower smaller trunk the pudic nerve ; and other branches are 

 given off by the several nerves before their union. 



Branches. Most of the offsets of the plexus are distributed out- 

 side the pelvis, and are met with in the dissection of the buttock 

 (pp. 109 et seq.) ; of these only the origin is to be seen now. The 

 branches of the plexus are : 



1. The great sciatic nerve. 



2. The small sciatic nerve. 



3. The superior gluteal nerve. 



4. Inferior gluteal nerve. 



5. The pudic nerve. 



6. Nerve to the obturator internus and superior gemellus. 



7. Nerve to the quadratus femoris and inferior gemellus. 



8. Nerve to the pyriformis. 



9. Perforating cutaneous nerve. 



a. The great sciatic nerve (fig. 150, gs) is the principal nerve of 



