SACEAL PLEXUS/ 727 



PLEXUS SACRAL1S. 



The sacral portion of the luinbo-sacral plexus is destined almost entirely 

 for the lower limb. It is usually formed by the anterior rami of a part of the 

 fourth lumbar nerve (n. furcalis), the fifth lumbar, the first, and parts of the 

 second, and third sacral nerves (n. bigeminus). 



Communications with the Sympathetic. Each of the nerves named is connected 

 to the lumbar or pelvic sympathetic by gray rami communicantes, as already described ; 

 and ivhite rami communicantes pass from the third and usually also from the second 

 or fourth sacral nerves to join the pelvic plexus of the sympathetic. 



Position and Constitution. The plexus is placed on the posterior wall of the 

 pelvis between the parietal pelvic fascia and the piriformis muscle. In front of it 

 are the pelvic colon, the hypogastric vessels, and the ureter. 



The plexus is constituted by the convergence of the nerves concerned towards 

 the inferior part of the greater sciatic foramen, and their union to form a broad 

 triangular band, the apex of which is continued through the greater sciatic 

 foramen below the piriformis muscle into the buttock, as the sciatic nerve. 

 From the anterior and posterior surfaces of this triangular band numerous small 

 branches arise, which are distributed to the parts in the neighbourhood of the 

 origin of the nerve. 



The sciatic nerve ends in the thigh by dividing into two large nerves, the tibial 

 (O.T. internal popliteal), and common peroneal (O.T. external popliteal). In many 

 cases these two nerves are distinct at their origin, and are separated sometimes by 

 fibres of the piriformis muscle. In all cases, on removal of the sheath investing 

 the sciatic nerve, the tibial and peroneal nerves can be traced up to the plexus, from 

 which they invariably take origin by distinct and separate roots. 



Formation. The descending branch of the fourth lumbar nerve (n. furcalis) 

 after emerging from the border of the psoas major muscle, medial to the obturator 

 nerve, divides behind the iliac vessels into anterior and posterior (ventral and dorsal) 

 parts, each of which joins a corresponding part of the fifth lumbar nerve. The 

 anterior ramus of the fifth lumbar nerve descends over the ala of the sacrum, and 

 divides into anterior and posterior parts, which are joined by the corresponding parts 

 of the fourth lumbar nerve. The two resulting trunks are sometimes called the 

 truncus lumbosacralis or lumbo- sacral trunk. The first and second sacral nerves pass 

 almost horizontally laterally from the anterior sacral foramina, and divide in 

 front of the piriformis into similar anterior and posterior parts. The third sacral 

 nerve (n. bigeminus) divides into superior and inferior parts. The inferior part is 

 concerned in forming the pudendal plexus. The superior part is directed laterally, 

 and slightly upwards, towards the second nerve, and does not separate into two 

 parts, but remains undivided. 



These trunks combine to form the sacral plexus, and its main subdivisions, 

 in the following way. Lying in apposition, and converging to the lower part 

 of the greater sciatic foramen, the posterior (dorsal) trunks of the fourth and fifth 

 lumbar nerves (lum bo-sacral trunk), and of the first and second sacral nerves, 

 combine to form the common peroneal nerve and the subordinate nerves which arise 

 from the posterior aspect of the plexus. The anterior (ventral) trunks of the fourth 

 and fifth lumbar nerves (lumbo-sacral trunk), and of the first and second sacral 

 nerves, together with that part of the third sacral nerve which is contributed to the 

 plexus, unite to form the tibial nerve and the subordinate nerves arising from the 

 front of the plexus. 



Of these nerves the fifth lumbar and first sacral are the largest; the others 

 diminishing in size as they are traced upwards and downwards. There is no 

 distinct demarcation between the sacral and pudendal plexuses. The second and 

 third sacral nerves (and in some cases the first sacral also) are concerned in the 

 formation of both plexuses. 



Branches. The nerves of distribution derived from the sacral plexus are 

 divided according to their origin into an anterior (ventral) and a posterior (dorsal) 



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