THE L UMB OS A CR+ 1 L PLEXUS \ 047 



the spinal column. In other cases, where the bone disease is more extensive and two or more 

 nerves are involved, a more diffused pain in the abdomen is complained of. A similar condition 

 is sometimes present in affections of the cord itself, as in tabes danaUs. 



Again, it must be borne in mind that the same nerves which supply the skin of the abdomen 

 supply jtjsq the muscles which constitute the greater part of the abdominal wall. Hence it follows 

 that any irritation applied to the peripheral terminations of the cutaneous branches in the 

 skin of the abdomen is immediately followed by reflex contraction of the abdominal muscles. 

 A good practical illustration of this may sometimes be seen in watching two surgeons examine 

 the abdomen of the same patient. One, whose hand is cold, causes the muscles of the abdom- 

 inal wall to contract at once and the belly to become rigid, and thus not nearly so suitable 

 for examination; the other, who has taken the precaution to warm his hand, examines the 

 abdomen without exciting any reflex contraction. The supply of both muscles and skin from 

 the same source is of importance in protecting the abdominal viscera from injury. A blow 

 on the abdomen, even of a severe character, will do no injury to the viscera if the muscles are 

 in a condition of firm contraction; whereas in cases where the muscles have been taken unawares, 

 and the blow has been struck while they were in a state of rest, an injury insufficient to produce 

 anv lesion of the abdominal wall has been attended with rupture of some of the abdominal con- 

 tents. The importance, therefore, of immediate reflex contraction upon the receipt of an injury 

 cannot be overestimated, and the intimate association of the cutaneous and muscular fibres in 

 the same nerve produces a much more immediate response on the part of the muscles to any 

 peripheral stimulation of the cutaneous filaments than would be the case if the two sets of fibres 

 were derived from independent sources. 



Again, the nerves supplying the abdominal muscles and skin are derived from the lower inter- 

 costal nerves and are intimately connected with the sympathetic supplying the abdominal viscera 

 through the lower thoracic ganglia from which the splanchnic nerves are derived. In conse- 

 quence of this, in rupture of the abdominal viscera and in acute peritonitis the muscles of the 

 belly wall become firmly contracted, and thus as far as possible preserve the abdominal contents 

 in a condition of rest. 



THE LUMBOSACRAL PLEXUS. 



The anterior primary divisions of the lumbar, sacral, and coccygeal nerves 

 form the lumbosacral plexus, the first lumbar nerve being frequently joined by a 

 branch from the twelfth thoracic. For descriptive purposes this plexus is usually 

 divided into three parts the lumbar, sacral, and pudenal plexuses. 



The Anterior or Ventral Divisions of the Lumbar Nerves (rami anteriores). 

 The anterior divisions of the lumbar nerves increase in size from above downward. 

 They are joined, near their origins, by gray rami communicantes from the lumbar 

 ganglia of the sympathetic cord. These consist of long, slender filaments, which 

 accompany the lumbar arteries around the sides of the bodies of the vertebra? 

 beneath the Psoas magnus muscle. Their arrangement is somewhat irregular; 

 one ganglion may give rami to two lumbar nerves, or one lumbar nerve may receive 

 rami from two ganglia. The first and second and sometimes the third and 

 fourth lumbar nerves are each connected with the lumbar part of the sympathetic 

 cord by a white ramus communicans. The nerves pass obliquely outward behind 

 the Psoas magnus, or between its fasciculi, distributing filaments to it and the 

 Quadratus lumborum muscles. The first three and the greater part of the fourth 

 are connected together in this situation by anastomotic loops, and form the lumbar 

 plexus. The anterior division of the fifth lumbar, joined with a branch from the 

 fourth, descends across the base of the sacrum to join the anterior division of the 

 first sacral nerve and assists in the formation of the sacral plexus. The cord 

 resulting from the union of the fifth lumbar and the branch from the fourth is 

 called the lambosacral cord (truncus lumbosacral is) (Figs. 769 and 774). 



The Lumbar Plexus (plexus lumbalis) (Figs. 768 and 769). The lumbar 

 plexus is formed by the loops of communication between the anterior divisions 

 of the four upper lumbar nerves. The plexus is narrow above, and often con- 

 nected with the last thoracic nerve by a slender branch. The plexus is bread 

 below, where it is joined to the sacral plexus by the lumbosacral cord. The 

 lumbar plexus is situated in the substance of the Psoas magnus muscle near 

 its posterior part, in front of the transverse processes of the lumbar vertebrae. 



