1006 



THE NERVOUS SYSTEM 



(figs. 765, 769). Sometimes the larger part of the fourth nerve may help to 

 form the trunk. It may receive fibres from the third lumbar nerve or be formed 

 entirely from the fifth. At its formation it is situated on the ala of the sacrum 

 under cover of the psoas. It descends into the pelvis, and, as it crosses the 

 anterior border of the ala of the sacrum, it emerges from beneath the psoas at the 

 medial side of the obturator nerve, from which it is separated by the ilio-lumbar 

 artery. It passes behind the common iliac vessels and unites with the first and 

 second sacral nerves, forming with them the upper trunk of the sacral plexus. 



4. SACRAL NERVES 



The anterior primary divisions of the upper four sacral nerves enter the pelvis 

 through the anterior sacral foramina and they diminish in size progressively from 

 above downward. The first sacral is the largest of the spinal nerves, the second 

 is slightly smaller than the first, while the third and fourth are relatively small. 

 The fifth sacral nerve is still smaller than the fourth; it enters the pelvis between 

 the sacrum and the coccyx. The anterior divisions of these nerves enter into the 

 formation of three parts of the lumbo-sacral plexus, the sacral, pudendal, and 

 coccygeal. 



Sacral Plexus 



The sacral plexus shows in its formation variations similar to those of the 

 lumbar plexus; hence there are also seven types of this plexus, three of them 

 belonging to the prefixed or proximal class, three to the postfixed or distal class, 

 and one to the ordinary class. The following tables show the range of variation 

 and the common arrangement in these classes: — 



Composition of the Nerves of the Sacral Plexus 

 Range of Variation 



Nerve. Proximal. 



Furcal 3 or 3, 4 L. 



Common peroneal (exter- 

 nal popliteal) 3, 4, 5 L. 1, 2 S. 



Tibial (internal popliteal) 3, 4, 5 L. 1, 2, S. 



Posterior femoral cutane- 

 ous (small sciatic) 5 L. 1, 2, 3 S. 



Common Composition 



Neuve. Proximal. Ordinary. Distal. 



Furcal 4 L. 4L. 4L. 



Common peroneal (exter- 

 nal popliteal 4, 5 L. 1, 2 S. 4, 5 L. 1, 2 S. 4, 5 L. 1, 2 S. 



Tibial (internal popliteal). 4, 5 L. 1, 2 S. 4, 5 L. 1, 2, 3 S. 4, L. 1, 2, 3, 4 S. 



Posterior femoral cutane- 

 ous (small sciatic) 1, 2, 3 S. 1, 2, 3 S. 2, 3 S. 



The ordinary tj^pe of sacral plexus is commonly formed by the smaller part of 

 the anterior division of the fourth lumbar nerve and the entire anterior division of 

 the fifth lumbar nerve, together with the first and parts of the second and third 

 sacral nerves. 



The plexus lies in the pelvis on tlu; anterior surface of the piriformis (fig. 765) 

 and behind the pelvic fascia and the l)ranches of the hypogastric (internal iliac) 

 artery. It is also dorsal to the coils of intestine, the lower part of the ilio-pelvic 

 colon lying in front of th(! left plexus, and the lower part of the ileum in front of the 

 right ])l(^xus. 



The liratichc's given off by this plexus are: — visceral, cutaneous, and muscular. 



Visceral branches are given off from the second, third, and fourth sacral 

 nerves to the pelvic viscera. 



The visceral branches correspond to white rami communicantes, through not joining the 

 sympathetic trunk. The branches from the second and fourth sacral nerves are inconstant. 



