ABDOMINAL CAVITY 403 



over the arteries and scraping away the fibres of the psoas 

 muscle. They join the lumbar nerves close to the inter- 

 vertebral foramina. 



The rami communicantes consist of two sets, viz., white and grey. 

 The white rami communicantes are composed of medullated fibres which 

 pass from the spinal nerves to the gangliated trunk. In the lumbar region 

 there are only two or at most three white rami, and they proceed from the 

 upper two or three lumbar nerves. The grey rami communicantes are 

 much more numerous, and are formed of fibres which stream out in an 

 irregular manner from the sympathetic trunk to the anterior rami of all the 

 lumbar nerves. 



The peripheral branches of distribution consist of a large 

 number of small filaments which arise irregularly from the 

 lumbar gangliated trunk, and pass medially, mainly to the 

 aortic plexus, but some of the lower twigs go to the hypo- 

 gastric plexus. 



Dissection. To bring the anterior rami of the lumbar nerves 

 into view, scrape away the remains of the psoas major muscle, 

 which was partially destroyed when the connecting sympathetic 

 twigs were followed backwards. An occasional branch, the 

 accessory obturator nerve, is liable to injury unless it is secured 

 at once. When present, it will be found descending along the 

 medial border of the psoas major. 



Lumbar Nerves. The anterior rami of the lumbar nerves 

 are five in number, they pass laterally through the substance 

 of the psoas major muscle. They increase in size from above 

 downwards, and each nerve is connected by one or more twigs 

 to the sympathetic trunk. Branches are given by the upper 

 four nerves to the quadratus lumborum and by the second, 

 third, and fourth to the psoas major. 



The first three lumbar nerves, with a part of the fourth, 

 unite in a loop-like manner to form the lumbar plexus, whilst 

 the remaining part of the fourth joins the fifth to form the 

 lumbo-sacral trunk. The fourth lumbar nerve is frequently 

 called the neruus furcalis, because it divides to take part in 

 the formation of both the lumbar and sacral plexuses. 



Plexus Lumbalis. The lumbar plexus is placed in front 

 of the transverse processes of the lumbar vertebrae, in the sub- 

 stance of the psoas major. Superiorly, it is usually connected 

 with the last thoracic nerve by a small twig which descends, in 

 the substance of the psoas, to the first lumbar nerve ; inferiorly, 

 it is brought into communication with the sacral plexus by 

 the branch of the fourth nerve which enters into the forma- 

 tion of the lumbo-sacral trunk. 

 IT 26a 



