606 THE NERVOUS SYSTEM. 
and fifth lumbar nerves (lumbo-sacral cord), and of the first and second sacral 
nerves, combine to form the 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 cord), 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 thus 
divided according to their origin into an anterior (ventral) and a posterior (dorsal) 
serves. Hach set comprises one of the two essential terminal parts—peroneal and 
tibial nerves—oft the great sciatic, and numerous smaller collateral branches. 
Anterior Branches. Posterior Branches. 
Tibial (internal popliteal) nerve Peroneal (external popliteal) nerve 
Muscular branches— Muscular branches— 
Nerves to hamstring muscles Nerves to short head of biceps 
ee quadratus femoris = pyriformis 
3 gemelli Superior gluteal nerve 
53 obturator internus Inferior gluteal nerve 
Articular branches (to hip-joint) Articular branches (to knee-joint) 
GREAT SCIATIC NERVE. 
The great sciatic nerve (n. ischiadicus)—It has already been shown how 
this nerve is formed. It comprises the two main nerves of the sacral plexus, bound 
together by an investing sheath, which contains, in addition to the peroneal and 
tibial nerves, a subordinate branch of each, the nerve to the hamstring muscles, 
from the tibial, and the nerve to the short head of the biceps flexor cruris, from 
the peroneal nerve. A thick band about half-an-inch in breadth is formed, con- 
sisting from within outwards of (1) nerves to the hamstring muscles, (2) tibial (internal 
popliteal), (5) peroneal (external popliteal), (4) nerve to short head of the biceps 
muscle. The great sciatic nerve extends ‘through the buttock and the back of 
the thigh. Forming the continuation of the sacral plexus, it enters the buttock by 
passing through the great sacro-sciatic foramen, in the interval between the 
pyriformis and superior gemellus. Concealed by the gluteus maximus muscle, it 
passes downwards to the thigh, accompanied by the sciatic artery, and the comes 
nervi ischiadici. It lies in the hollow between the great trochanter of the femur 
and the tuberosity of the ischium, and enters the thigh beneath the fold of the 
nates and the lower border of the gluteus maximus. At this spot it is com- 
paratively superficial, lying in the angle between the edge of the gluteus maximus 
above and externally, and the origins of the hamstring muscles internally. In the 
thigh it is placed upon the adductor magnus beneath the hamstring muscles, 
and it terminates at a variable point by dividing into the tibial and peroneal 
nerves. As already stated, these two nerves may be separate from their origins, 
and their separation may occur at any point between the great sacro-sciatic foramen 
and the upper part of the popliteal space. 
THE NERVES OF DISTRIBUTION FROM THE SACRAL PLEXUS. 
These are divisible into two series—collateral and terminal branches. Each 
subdivision consists of a series of anterior (ventral) and posterior (dorsal) trunks. 
Collateral Branches. — The anterior branches are (a) muscular branches 
(to the quadratus femoris, gemelli, obturator internus, and hamstring muscles) ; 
and ()) articular branches (to the hip-joint). These nerves all arise from the 
anterior aspect of the sacral plexus. 
