720 THE KEKVOUS SYSTEM. 



and inferior branches. The superior branch of the first nerve (which may be joined 

 by the branch from the twelfth thoracic nerve) forms two nerves, the ilio-hypogastric 

 and ilio-inguinal. The inferior branch of the first joins the superior branch of the 

 second nerve, to produce the genito -femoral nerve (O.T. genito-crural). The inferior 

 branch of the second" nerve, the whole of the third, and that part of the fourth 

 nerve engaged in the constitution of the plexus divide each into two unequal parts 

 smaller anterior and larger posterior parts. The smaller anterior portions combine 

 together to form the obturator nerve, which is thus formed by the second, third, and 

 fourth lumbar nerves. The root from the second nerve is not always present. The 

 larger posterior portions of the same nerves combine together to form the femoral 

 nerve (O.T. anterior crural). From the posterior aspect of the posterior parts of the 

 second and third nerves the lateral cutaneous nerve of the thigh (O.T. external 

 cutaneous) arises. The nerves also provide, near their origins, irregular muscular 

 branches for the psoas and quadratus lumborum muscles. The following is a list 

 of the nerves which spring from the lumbar plexus (Figs. 624 and 625) : 



(1) Muscular branches to the quadratus (4) Genito-femoral. 



lumborum and psoas muscles. (5) Lateral cutaneous. 



(2) Ilio-hypogastric. (6) Obturator. 



(3) Ilio-inguinal. (7) Femoral. 



Muscular Branches. The nerves to the quadratus lumborum muscle arise 

 independently from the first three or four lumbar nerves (and sometimes also from 

 the twelfth thoracic nerve). The nerves to the psoas muscles arise from the second 

 and third lumbar nerves, with additions, in some cases, from the first or fourth. 

 They are often associated in their origin with the nerve to the iliacus from the 

 femoral nerve. The psoas minor, when present, is innervated by the first or 

 second lumbar nerve. 



The ilio-hypogastric and ilio-inguinal nerves closely resemble, in their course 

 and distribution, the lower thoracic nerves, with which they are in series. 



N. Iliohypogastricus. The ilio-hypogastric nerve is the highest branch of 

 the first lumbar nerve. It receives fibres also from the twelfth thoracic, when 

 that nerve communicates with the first lumbar nerve. After traversing the psoas 

 muscle obliquely, it appears at its lateral border, on the surface of the quadratus 

 lumborum and behind the kidney. It courses through the loin, lying between the 

 transversus and obliquus abdominis internus muscles, above the crest of the ilium. 

 About an inch in front of the anterior superior spine it pierces the obliquus internus, 

 and continues its course in the groin beneath the aponeurosis of the obliquus ex- 

 ternus. It finally becomes cutaneous in the anterior abdominal wall, by piercing 

 the aponeurosis of the obliquus externus about an inch and a half above the sub- 

 cutaneous inguinal ring (Fig. 623, p. 715). 



Its branches are (1) muscular to the muscles of the abdominal wall ; and (2) 

 cutaneous branches, two in number. The lateral cutaneous branch corresponds 

 with the lateral branch of an . intercostal nerve, and, after piercing the obliquus 

 internus and obliquus externus, becomes cutaneous just above the iliac crest, below 

 and behind the iliac branch of the last thoracic nerve. It is small, and may be; 

 absent. It is distributed to the skin over the superior part of the lateral side of 

 the buttock, in continuity with the cutaneous branch of the posterior ramus olf 

 the first lumbar nerve. The anterior cutaneous branch is the anterior terminal 

 branch of the nerve. It supplies the skin of the anterior abdominal wall belo\v 

 the level of the last thoracic nerve and above the os pubis. 



N. Ilioinguinalis. The ilio-inguinal nerve is the second branch given of 

 from the first lumbar nerve. It also may receive fibres from the last thoracic 

 nerve. Not infrequently the ilio-hypogastric and ilio-inguinal nerves are repre 

 sen ted for a longer or shorter part of their course by a single trunk. Wher 

 separate the nerve takes a course similar to that of the ilio-hypogastric nerve 

 but at a lower level, as far as the anterior abdominal wall. It then pierces th< 

 obliquus internus farther forward and lower down than the ilio-hypogastric 

 and coursing forwards beneath the aponeurosis of the obliquus externus, jus 



