CHAP, ix.] NERVOUS SYSTEM AND ORGANS OF SENSE. 281 



and adjacent dorsal border of the index, and another branch supplies 

 the adjacent sides of the index and third digits, and the radial side of 

 the fourth digit, also giving off a branchlet to join the dorsal branch 

 of the ulnar nerve. 



The posterior interosseous nerve passes through the supinator 

 brevis to the back of the fore-arm, where it divides, and is dis- 

 tributed to the muscles of that region. 



The ventral primary divisions of the DORSAL, or THORACIC NERVES, 

 pass out in the intercostal spaces along with the intercostal blood- 

 vessels, the last passing along behind the last rib. They soon dip 

 between the internal and external intercostal muscles, and each 

 gives off a twig to the skin at a point about midway between the 

 vertebral column and the sternum. The more posterior of the thoracic 

 nerves enter the abdominal wall, and go to the margin of the rectus, 

 passing, on their way, between the internal oblique and the trans- 

 versalis. They enter the rectus and send small branches to the 

 skin. The last dorsal nerve sends back a branch which unites with 

 the first lumbar, and so joins in what is called the lumbar plexus. 

 Each thoracic spinal nerve sends a short twig, inwards and down- 

 wards, to join the sympathetic. 



19. The ventral primary divisions of the LUMBAR NERVES are 

 larger than those of the dorsal nerves. They severally give off a 

 filament to the sympathetic, and then unite in loops to form a 

 contimious, complex interlacement of branches, called the LUMBAR 

 and SACRAL PLEXUSES, whence come the nerves of the HIND-LIMB. 

 The lumbar plexus lies on the ventral aspect of the lumbar transverse 

 processes, and is formed by the fourth, fifth, sixth, and seventh lumbar 

 nerves. The fourth lumbar nerve, after giving off a branch which 

 divides into the ilio-hypogastric and ilio-inymnal nerves (Fig. 133, He 

 and ill), sends a branch backwards, which joins the root of the fifth 

 lumbar nerve. From this junction the long genito-crural nerve (gc) is 

 given off, and shortly afterwards that called the external cutaneous. The 

 trunk then bifurcates, its two branches joining the two branches into 

 which the sixth lumbar nerve divides. The larger pair of branches 

 thus joining, give origin to the anterior crural nerve (ac), while a 

 small twig to the psoas muscle is given off by that root of the 

 anterior crural which is contributed by the fifth lumbar nerve. The 

 other two branches from the fifth and sixth nerves unite to form a 

 branch which gives off the obturator nerve (ob), and then passes 

 backwards to join the seventh lumbar nerve, the thick trunk result- 

 ing from their junction being called the lumbo-sacral cord, and con- 

 stituting the main root of the great sciatic nerve. 



It is called " lumbo-sacral " because the nerves which come out of 

 the anterior sacral foramina are reckoned as forming by themselves 

 a sacral plexus, which plexus is placed in communication with the 

 lumbar plexus by means of this " lumbo-sacral cord.' 7 



From the outer side of this " cord" the piidic and gluteal nerves 

 are given forth. The two sacral nerves unite together after the 

 second sacral has given off some branches to the tail (cd) to form 



