802 THE XERVOUS SYSTEM 



LU3IBAR NERVES 



The anterior divisions of the five luml^ar nerves increase progressively in size 

 from above downwards, the first lumbar being about the size of an intercostal nerve, 

 and the fifth a nerve of considerable magnitude. The first, second, third, and part 

 of the fourth nerves unite to form the lumbar plexus. The remainder of the fourth 

 unites with the fifth to form the luml)o-sacral cord, which passes to the sacral plexus. 

 Each lumbar nerve is connected to the gangliated cord of the sympathetic by rami 

 communieantes. These rami communicantes reach the lumbar nerves by accom- 

 panying the lumbar arteries through the fibrous arches from which the psoas takes 

 origin. 



LUMBAR PLEXUS 



The lumbar plexus is placed among the deeper fibres of the psoas, and is con- 

 stituted as follows: the first lumbar nerve is joined by a communicating twig from 

 the tAvelfth thoracic and divides into two branches; one of these is the common 

 origin of the ilio-hypogastric and ilio-inguinal nerves; the other branch unites with 

 a branch of the second lumbar to form the genito-crural nerve. The second, third, 

 and fourth lumbar nerves divide into anterior and posterior divisions. A part of 

 the anterior division of the second lumbar nerve enters into the composition of the 

 genito-crural nerve as above mentioned; the remainder of the anterior division of 

 the second nerve unites with the anterior division of the third and part of the fourth 

 nerve to form the obturator nerve. The remainder of the anterior division of the 

 fourth nerve enters the lumbo-sacral cord. The fourth lumbar nerve, therefore, 

 takes part in the formation of l^oth the lumbar and sacral plexuses; and it is known 

 as the "nervus furcalis." The posterior divisions of the second, third, and fourth 

 nerves end chiefly in two large nerves — the external cutaneous and the anterior 

 crural. The external cutaneous arises from the second and third nerves. The 

 anterior crural springs from the second, third, and fourth nerves. A part of tht- 

 posterior division of the fourth nerv^e enters the lumbo-sacral cord. The large ter- 

 minal ])ranches above described are given off within the substance of the psoas; 

 three of these, the ilio-hypogastric, ilio-inguinal, and external cutaneous, appear at 

 the outer border of the psoas, l^etween that muscle and the quadratus lumborum: 

 one, the genito-crural, pierces the anterior fibres of the psoas and appears in front 

 of the muscle; another l^ranch, the obturator, appears at the inner border of tin- 

 psoas close to the lower margin of the fifth lumbar vertebra; lastly, the anterior 

 crural nerve, the largest branch of the plexus, runs downwards in the interval 

 between the psoas and iliacus. AMthin the psoas small twigs are given oft' l\v the 

 lumbar nerves, which end in the nmscular substance of the psoas and quadratus 

 lumljorum. 



Branches. — The nerves to the psoas and quadratus Ivmhorum from the first and 

 second luml)ar nerves arise in the sul)stance of the psoas. 



The ilio-hypogastric and ilio-inguinal nen'es are in some cases represented 

 by a common trunk Avhich arises from the first lumbar. More commonly they 

 exist as two se})arate nerves, the upper and larger nerve being ilio-hypogastric: tlu' 

 lower and smaller, ilio-inguinal. Taken together they correspond in many respects 

 to a lower intercostal nerve. They run for a considera1)le part of their course 

 between the transversalis and internal oblique, giving twigs to both these muscles. 

 The ilio-hypogastric gives off a lateral cutaneous, which is called the iliac branch, 

 and terminates in an anterior cutaneous which is termed the hypogastric branch. 



(1) The ilio-hypogastric nerve is a l)ranch of the first lumbar. It ai»})ears at 

 the outer l^order of the psoas and crosses the quadratus lumliorum ol)li<iuely \o 

 reach the crest of the ilium. In this part of its course it is surrounded by the 

 extra-peritoneal fat. It then pierces the transversalis and runs along the crest of 

 th(^ ilium between that nmscle and the internal ol)li(]ue. and. about two and a half 

 inches behind the anterior superior spine of the ilium, divides into an iliac and 



