THE LUMBAR PLEXUS. 851 



divides into the ilio-hypogastric and ilio-inguinal ; the lower one descends and 

 subdivides into two branches, an anterior and a posterior. The second lumbar 

 nerve sends a branch to join with the anterior of the two preceding, to form the 

 genito-crural nerve ; the rest of the nerve then receives the posterior of the two 

 above mentioned, and proceeds downward, giving off an external, a middle, and 

 an internal branch. The third lumbar nerve gives off three branches, known as 

 dorsal, middle, and ventral. The fourth lumbar nerve also divides into three 

 branches, known as anterior, posterior, and inferior. These various subdivisions 

 now unite as follows : The external from the second joins the dorsal from the 

 third to form the external cutaneous nerve. The middle branches from the second 

 and third together with the posterior from the fourth, unite to form the anterior 

 crural nerve ; while the remaining (internal and ventral) branches of the second 

 and third lumbar nerves unite with the anterior of the fourth to form the obturator 

 nerve. The remainder of the anterior division of the fourth nerve passes down 

 to communicate with the fifth lumbar nerve. The accessory obturator, when it 

 exists, is formed by a small branch from the third nerve joining with a small 

 branch from the fourth. 



From this arrangement it follows that the ilio-hypogastric and ilio-inguinal 

 are derived entirely from the first lumbar nerve ; the genito-crural from the first 

 and second nerves ; the external cutaneous from the second and third ; the ante- 

 rior crural and obturator by fibres derived from the second, third, and fourth ; 

 and the accessory obturator, when it exists, from the third and fourth. 



The branches of the lumbar plexus are the 



Ilio-hypogastric. Anterior crural. 



Ilio-inguinal. Obturator. 



Genito-crural. Accessory obturator. 

 External cutaneous. 



The nio-hypogastric Nerve (superior musculo-cutaneous) arises from the firsi 

 lumbar nerve. It emerges from the outer border of the Psoas muscle at its upper 

 part, and crosses obliquely in front of the Quadratus lumborum to the crest of the 

 ilium. It then perforates the Transversalis muscle at its posterior part, near the 

 crest of the ilium, and divides between it and the Internal oblique into two 

 branches, iliac and hypogastric. 



The iliac branch pierces the Internal and External oblique muscles imme- 

 diately above the crest of the ilium, and is distributed to the integument of the 

 gluteal region, behind the lateral cutaneous branch of the last dorsal nerve (Fig. 

 509). The size of this nerve bears an inverse proportion to that of the cutaneous 

 branch of the last dorsal nerve. 



The hypogastric branch (Fig. 505) continues onward between the Internal 

 oblique and Transversalis muscles. It then pierces the Internal oblique, and 

 near the middle line perforates the aponeurosrs of the External oblique, about an 

 inch above and a little to the outer side of the external abdominal ring, and is 

 distributed to the integument of the hypogastric region. 



The ilio-hypogastric nerve communicates with the last dorsal and ilio-inguinal 

 nerves. 



The Ilio-inguinal Nerve (inferior musculo-cutaneous), smaller than the pre- 

 ceding, arises with it from the first lumbar nerve. It emerges from the outer 

 border of the Psoas just below the ilio-hypogastric, and, passing obliquely across 

 the Quadratus lumborum and Iliacus muscles, perforates the Transversalis near 

 the fore part of the crest of the ilium, and communicates with the ilio-hypogastric 

 nerve between that muscle and the Internal oblique. The nerve then pierces the 

 Internal oblique, distributing filaments to it ; and, accompanying the spermatic 

 cord through the inguinal canal, it escapes at the external abdominal ring, and is 

 distributed to the integument of the upper and inner part of the thigh, and to the 

 scrotum in the male and to the labium in the female. The size of this nerve is in 



