<J44 THE NERVOUS SYSTEM 



ceive 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 de- 

 scends into the pelvis, and, as it crosses the anterior border of the ala of the sacrum, it 

 emerges from beneath the psoas at the inner 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. 



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 

 downwards. 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. ORDINARY. DISTAL. 



Furcal 3 or 3, 4 L. 4 L. 4, 5 or 5 L. 



Common peroneal (ex- 

 ternal popliteal) ... 3, 4, 5 L. 1,2S. 4, 5L. 1,2S. 4, 5 L. 1,2,38. 



Tibial (internal pop- 

 liteal) 3,4,5L. 1,2S. 4,5L. 1,2,3S. 4,5L. 1,2,348. 



Posterior femoral cu- 

 taneous (small sci- 

 atic) 5L. 1,2,38. 5L. 1,2,3,48. 5L. 1,2,3,48. 



COMMON COMPOSITION 

 NERVE. PROXIMAL. ORDINARY. DISTAL. 



Furcal 4 L. 4 L. 4 L. 



Common peroneal (ex- 

 ternal popliteal) ... 4, 5 L. 1,2 S. 4, 5 L. 1, 28. 4, 5 L. 1, 2 S. 



Tibial (internal pop- 



"teal) 4,5L. 1, 2 S. 4,5L. 1,2,38. 4, 5 L. 1, 2, 3, 4 S. 



Posterior femoral cu- 

 taneous (small sci- 

 atic) 1,2, 3 S. 1,2,38. 2,38. 



The ordinary type 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 the anterior surface of the piriformis (fig. 692) and 

 behind the pelvic fascia and the branches of the hypogastric (internal iliac) artery. 



