NERVOUS SYSTEM 625 



dorsal muscles of the spine, and their long ventral branches run 

 between the ribs, supplying the intercostal and, further back, the 

 abdominal muscles. 



The Sacral Plexus is formed by the Spinal Nerves in the 

 pelvic region, and may be conveniently divided into (A) a 

 Crural or Lumbar, (B) a Sciatic, and (C) a Pudic portion. The 

 first (A) is composed of from 2 to 4, but generally of 3 nerves, 

 the foremost of which sends a long branch to the abdominal 

 muscles, while the hindmost, n. furcalis, leaving the spinal column, 

 as a rule, between the 2 last lumbo-sacral vertebrae, divides one-half 

 going to the Sciatic portion. From the Crural portion spring 

 several branches forming 3 groups (1) those that serve the m. 

 sartorius, m. ambiens and some other muscles of the leg ; (2) the 

 n. obturatorius, supplying the m. obturator and its accessories, as 

 well as the m. adductor magnus ; and (3) a long nerve which runs 

 down the median side of the thigh to the inner side of the knee, 

 supplying the latter and passing subcutaneously down the median 

 side of the leg. This nerve is almost peculiar to Birds, occurring 

 beside only in Crocodiles and Monotremes. 



The Sciatic portion (B) generally consists of 5 or 6 nerves, 

 which leave the pelvis as a thick stem, passing close behind the anti- 

 trochanter through the ischio-iliac foramen, where a strong branch 

 separates itself from the hinder side of the common stem to supply 

 most of the adductor muscles of the thigh and leg. From the 

 main stem branches are given off to the ilio-femoral, ilio-tibial, ilio- 

 fibular and ischio-femoral muscles. The rest of the stem continues 

 as the Sciatic nerve, accompanied by the great arteries and veins 

 on the posterior side of the thigh, and below the knee invariably 

 divides into 3 branches, the first of which (1) is the strongest, and 

 passes with the tendon of the ilio-fibular muscle through the peculiar 

 tendinous pulley on the side of the fibula, whereupon it splits into 

 the superficial and deep peroneal nerves, to supply the extensor 

 and peroneal muscles of the foot and toes ; the median branch (2) 

 soon breaks up into a number of nerves for the deep flexor muscles 

 of the toes and the inner and middle portion of the m. gastro- 

 cnemius ; while the third (3) innervates the outer head of the 

 gastrocnemius and the rest of the flexor muscles of the toes. As 

 before stated, the Sciatic portion (B) receives one-half of the n. 

 furcalis from the Crural (A), while its hindmost spinal stem, leaving 

 the spinal column, in most birds, between or just below the hind- 

 most of the 2 primary sacral vertebrae (see Pelvis under SKELETON), 

 sends a branch to the Pudic portion (C), which is composed of the 

 post-ischiadic spinal nerves. These are partly imbedded in the 

 substance of the KIDNEYS, and run obliquely outwards, forming 

 many anastomoses with one another, especially on the hinder parts of 

 the ischium and pubic bone. This portion chiefly innervates the 



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