THE LIMBS. 297 



median below the insertion of the coraco-brachialis ; this 

 communication is frequently seen in lower primates ; its meaning 

 is not known. (3) A communication between median and ulnar 

 in the forearm is also common and is seen constantly in some 

 primates. The communicating branch passes with the deep 

 branch of the ulnar nerve to the palm. 



Nerve Supply of the Lower Limb. — Usually ten segments 

 contribute to the nerve supply of the lower limb — the 12 th 

 dorsal to the 4th sacral (Fig. 241). The sensory nerves are 

 derived from these segments ; the motor nerves begin at the 1st 

 lumbar segment and end at the 3rd sacral. There is a consider- 

 able variation in the number of body segments or vertebrae to 

 which the lower limb is attached; usually it is the 25th vertebra 

 which becomes the 1st sacral, but it may be the 26 th or 24th 

 (page 144). Of these three forms the first is the normal type 

 (25th); the second the post-fixed type (26th), the third the 

 pre-fixed type (24th). There is even a greater variation in the 

 segments which contribute nerves to the limb ; the normal motor 

 segments are the 1st lumbar to the 3rd sacral ; in the post-fixed 

 type (more common than the next) the motor segments commence 

 at the 2nd lumbar and cease at the 4th sacral ; in the pre-fixed 

 type the motor segments commence at the 12th dorsal and end 

 at the 2nd sacral. The spinal nerve which bifurcates and joins 

 both lumbar and sacral plexuses is known as the nervus furcalis. 

 In the normal type it is the 4th lumbar; in the pre-fixed type 

 it is the 3rd lumbar ; in the post-fixed type the 5th lumbar. 



The nervus bigeminus, normally the 4th sacral, may also vary 

 in a corresponding manner. 



The nerves to the extensor surface of the lower limb, the 

 anterior crural, external popliteal, etc., represent the dorsal 

 divisions of lateral cutaneous nerves (Fig. 238). The nerves to 

 the adductor and flexor aspects, the obturator and internal 

 popliteal, represent the ventral divisions. In a considerable 

 number of individuals, the dorsal division (external popliteal) 

 and ventral (internal popliteal) of the great sciatic separate in 

 the pelvis, the external popliteal perforating the pyriformis. 



The muscles of the lower extremity are supplied in the normal 

 type from the following segments : 



Motor Segments. — 1st Lumbar. Psoas and Iliacus. 



