THE LUMBAR .Y7?T7>. 849 



fairly equidistant from each other. The sixth and seventh supply the skin over the " pit of the 

 stomach: " the eighth corresponds to about the position of the middle linea transversa ; the 

 tenth to the umbilicus : and the ilio-hypogastric supplies the skin over the pubes and external 

 abdominal ring. There are several points of surgical importance about the distribution of these 

 nerves, and it is important to remember their origin and course, for in many diseases affecting 

 the nerve-trunks at or near the origin the pain is referred to their peripheral terminations. 

 Thus in Pott's disease of the spine children will often be brought to the surgeon suffering from 

 pain in the belly. This is due to the fact that the nerves are irritated at the seat of disease as 

 they issue from the spinal canal. When the irritation is confined to a single pair of nerves, the 

 sensation complained of is often a feeling of constriction, as if a cord were tied round the abdo- 

 men ; and in these cases the situation of the sense of constriction may serve to localize the 

 disease in the spinal column. In other cases, where the bone disease is more extensive and two 

 or more nerves are involved, a more general diffused pain in the abdomen is complained of. A 

 similar condition is sometimes present in affections of the cord itself, as in tabes dorsalis. 



Again, it must be borne in mind that the same nerves which supply the skin of the abdomen 

 supply also the planes of muscle which constitute the greater part of the abdominal wall. Hence 

 it follows that any irritation applied to the peripheral terminations of the cutaneous branches in 

 the skin of the abdomen is immediately followed by reflex contraction of the abdominal muscles. 

 A good practical illustration of this may sometimes be seen in watching two surgeons examine 

 the abdomen of the same patient. One. whose hand is cold, causes the muscles of the abdominal 

 wall to at once contract and the belly to become rigid, and thus not nearly so suitable for examina- 

 tion : the other, who has taken the precaution to warm his hand, examines the abdomen with- 

 out exciting any reflex contraction. The supply of both muscles and skin from the same source 

 is of importance in protecting the abdominal viscera from injury. A blow on the abdomen, 

 even of a severe character, will do no injury to the viscera if the muscles are in a condition of 

 firm contraction : whereas in cases where the muscles have been taken unawares, and the blow 

 has been struck while they were in a state of rest, an injury insufficient to produce any lesion of 

 the abdominal wall has been attended with rupture of some of the abdominal contents. The 

 importance, therefore, of immediate reflex contraction upon the receipt of an injury cannot be 

 overestimated, and the intimate association of the cutaneous and muscular fibres in the same 

 nerve produces a much more immediate response on the part of the muscles to any peripheral 

 stimulation of the cutaneous filaments than would be the case if the two sets of fibres were derived 

 from independent sources. 



Again, the nerves supplying the abdominal muscles and skin derived from the lower inter- 

 costal nerves are intimately connected with the sympathetic supplying the abdominal viscera 

 through the lower thoracic ganirlia from which the splanchnic nerves are derived. In con- 

 sequence of this, in laceration of the abdominal viscera and in acute peritonitis the muscles of the 

 belly-wall become firmly contracted, and thus as far as possible preserve the abdominal contents 

 in a condition of rest. 



THE LUMBAR NERVES. 



The lumbar nerves are five in number on each side. The first appears between 

 the first and second lumbar vertebra?, and the last between the last lumbar and the 

 base of the sacrum. 



The roots of the lower lumbar (and upper sacral) nerves are the largest, and their 

 filaments the most numerous, of all the spinal nerves, and they are closely aggre- 

 gated together upon the lower end of the cord. The anterior roots'are the 

 smaller, but there is not the same disproportion between them and the posterior 

 roots as in the cervical nerves. The roots of these nerves have a vertical direction, 

 and are of considerable length, more especially the lower ones, since the spinal 

 cord does not extend beyond the first lumbar vertebra. The roots become joined 

 in the intervertebral foramina, and the nerves so formed divide at their exit into 

 two divisions, posterior and anterior. 



Posterior Divisions of the Lumbar Nerves. 



The posterior divisions of the lumbar nerves (Fig. 502) diminish in size from 

 above downward; they pass backward beneath the transverse processes, and 

 divide into internal and external branches. 



The internal branches, the smaller, pass inward close to the articular 

 processes of the vertebra?, and supply the Multifidus spina? and Interspinales 

 muscles. 



The external branches supply the Erector spina? and Intertransverse muscles. 

 From the three upper branches cutaneous nerves are derived which pierce the 

 aponeurosis of the Latissimus dorsi muscle and descend over the back part of the 



54 



