THE SACRAL OR SCIATIC PLEXUS. 605 
The accessory obturator nerve (n. obturatorius accessorius, n. accessorius anterioris cruralis, 
Winslow) is only occasionally present (29 per cent, Hisler). It arises from the third, or third and 
fourth lumbar nerves, between the roots of the obturator and anterior crural nerves. Associating 
itself with the obturator, from which, however, it is quite separable, it appears in the abdomen 
at the inner side of the psoas muscle, and coursing over the pelvic brim behind the external iliac 
vessels, it leaves the obturator nerve, and enters the thigh in front of the pubis. 
In the thigh, behind the femoral vessels, it ends usually in three branches: a nerve which 
replaces the branch from the anterior crural to the pectineus, a nerve to the hip joint, and a 
nerve which communicates with the superficial part of the obturator nerve. In some cases it 
only supplies the nerve to the pectineus ; more rarely it is of considerable size, and reinforces the 
obturator nerve in the innervation of the adductor muscles. 
The accessory obturator nerve was first described by Winslow as the n. accessorius anterioris 
eruralis. Schmidt later described it in great detail, and gave it the name it now bears. It is 
more closely associated with the anterior crural than with “the obturator. Its origin is behind 
the roots of the obturator: it is separated, like the anterior crural, from the obturator by the 
pubic bone, and its chief branch to the pectineus muscle replaces the normal branch from the 
anterior crural nerve. On the other hand, for a part of its course it accompanies the obturator, 
and in rare cases it may replace branches of that nerve. 
THE SACRAL OR SCIATIC PLEXUS. 
The sacral or sciatic portion of the lumbo-sacral plexus is destined almost 
entirely for the lower imb. It is usually formed by the anterior primary divisions 
of a part of the fourth lumbar nerve (n. furcalis), the fifth lumbar, the first, 
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 white rami communicantes pass usually from the third and sometimes also from the 
second and fourth sacral nerves to join the pelvic plexus of the sympathetic. 
Position and Constitution.—The plexus is placed on the back wall of the 
pelvis between the parietal pelvic fascia and the pyriformis muscle. In front of it 
are the pelvic colon, the internal iliac vessels, and the ureter. 
The formation of the plexus occurs by the convergence of the nerves concerned 
towards the lower part of the great sacro-sciatic foramen, and their union to form 
a broad triangular band, the apex of which is continued through the great sacro- 
sciatic foramen below the pyriformis muscle into the buttock as the great 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 great sciatic nerve ends in the thigh by dividing into two large nerves, the 
tibial (internal popliteal), and peroneal (external popliteal). In many cases these 
two nerves are distinct from their origin, and are separated sometimes by fibres 
of the pyriformis muscle. In all cases on removal of the sheath investing the 
ereat 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. 
The descending branch of the fourth lumbar nerve (n. furcalis) after emerging 
from the inner border of the psoas muscle internal 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 
primary division 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 
lumbo-sacral cord. The first and second sacral nerves pass aliost horizontally out- 
wards from the anterior sacral foramina, and divide in front of the pyriformis into 
similar anterior and pesterior parts. The third sacral nerve (n. bigeminus) divides 
into upper and lower parts. The lower part is concerned in forming the pudendal 
plexus. The upper part is directed outwards, and slightly upwards, towards the 
preceding nerve, and does not separate into two parts, but remains undivided. 
These trunks combine to form the sciatic or sacral plexus, and its main sub- 
divisions, in the following way. Lying in apposition, and converging to the lower 
part of the great sacro-sciatic foramen, the posterior (dorsal) trunks of the fourth 
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