780 THE NERVOUS SYSTEM. 



The last dorsal is larger than the other dorsal nerves. Its anterior division 

 runs along the lower border of the last rib, and passes under the external arcuate 

 ligament of the Diaphragm. It then runs in front of the Quadratus lumborum, 

 perforates the Transversalis, and passes forward between it and the Internal 

 oblique, to be distributed in the same manner as the lower intercostal nerves. It 

 communicates with the ilio-hypogastric branch of the lumbar plexus, and is fre- 

 quently connected with the first lumbar nerve by a slender branch, the dor si- 

 lumbar nerve, which descends in the substance of the Quadratus lumborum. It 

 gives a branch to the Pyramidalis muscle. 



The lateral cutaneous branch of the last dorsal is remarkable for its large size; 

 it perforates the Internal and External oblique muscles, passes downward over the 

 crest of the ilium in front of the iliac branch of the ilio-hypogastric (Fig. 422), 

 and is distributed to the integument of the front part of the gluteal region, some of 

 its filaments extending as low down as the trochanter major. It does not divide 

 into an anterior and a posterior branch, like the other lateral cutaneous branches 

 of the intercostal nerves. 



Surgical Anatomy. The lower seven intercostal nerves and the ilio-hypogastric from the 

 first lumbar nerve supply the skin of the abdominal wall. They run downward and inward 

 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 ganglia 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 vertebrae, and the last between the last lumbar and the 

 base of the sacrum. 



The roots of the lumbar nerves are the largest, and their filaments the most 

 numerous, of all the spinal nerves, and they are closely aggregated together upon 

 the lower end of the cord. The anterior roots are the smaller, but there is not the 



