768 A MANUAL OF ANAtOMY 



four lumbar vertebrae, by tendinous slips which lie behind the slips 

 of origin. 



Nerve-supply. — ^The subcostal nerve and the anterior primary 

 divisions of the first two lumbar nerves. 



Action. — Acting from its origin the muscle depresses and fixes 

 the last rib, and is therefore a muscle of inspiration, inasmuch as 

 it is auxiliary to the diaphragm. In depressing the last rib the 

 muscle is also capable of producing lateral flexion of the vertebral 

 column. Acting from the last rib it will produce lateral flexion of 

 the pelvis. 



The muscle is encased in a sheath, the anterior wall of which is 

 formed by the anterior lamina of the lumbar aponeurosis, and the 

 posterior wall by the middle lamina. 



Lumbar Plexus. — ^The lumbar plexus is situated deeply in front 

 of the transverse processes of the first three lumbar vertebrae, 

 where it lies in the psoas magnus. It is formed by the anterior 

 primary divisions of the first three lumbar nerves and the greater 

 part of that of the fourth. In addition, the anterior primary 

 division of the first lumbar is usually reinforced by a small com- 

 municating branch from the subcostal nerve, called the dorso- 

 lumbar nerve. The nerves coricerned in the lumbar plexus first 

 furnish the following branches : (i) the first gives twigs to the 

 psoas parvus when present ; (2) the first and second supply branches 

 to the quadratus lumborum ; (3) the second and third give branches 

 to the psoas magnus ; and (4) the upper two or three furnish white 

 rami communicantes to the lumbar sympathetic gangliated cord. 

 The mode of formation and branches of the plexus are as follows : 

 The first lumbar, having been, as a rule, reinforced by the dorso- 

 j 'umbar from the subcostal, furnishes, from above downwards, the 

 .^.- rnuscie^ff.astric and inguinal, and then it descends to join a branch 

 ilio-hyp^^. _ _ _ThiTljPu second, third, and that part of the fourth 

 ^^^1,* second. x_ itame^'v. break up into a small anterior or 

 which enters into the plex\nracic ai/iorsal division. The descending 

 ventral, and a large posterior or'^J^Y "^'^ from the ventral division of 

 branch from the first joins a brancn^^^^^^^^nerve, which arises next in 

 the second to form the genito-femoral i-minenious nerve arises by two 

 order to the inguinal. The external cutafftr septond and third. The 

 roots from the dorsal divisions of the sei^^°^ ^' the dorsal divisions 

 anterior femoral nerve arises by three roots from!,tw ^ third being the 

 of the second, third, and fourth, the root from thethde roots from the 

 largest Ihe obturator nerve arises usually by thre^ ^^^t the root from 

 ventral divisions of the second, third, and fourth '■ ^^^P^ nerve, when 



nresenr".ri ""^^ ^! ''^''"*- ^^' ^^^^^^«^y obtm-at.'^'prth, which are 

 present, arises by two roots from the third and fou i^ and main 

 interposed between the roots of the anterior crui^^ ^^ 

 obturator nerves. ^"^"^ acrai ^^ follows • 



The branches of the lumbar plexus are accordinfflv^^^'"^ ^^"^bar. 



I. Muscular to psoas parvus (when present), from fir^^ ^nd second 

 ^^^2.Juscular to quadratus lumbofmn, from first \ 



