628 CIRCULATION OF BLOOD AND LYMPH. 



nerves of the limbs have been demonstrated frequently, as already- 

 explained. Whether or not these fibers also pass through the 

 sympathetic system, following the same general course as the 

 vasoconstrictors, has not been shown conclusively. The most 

 definite work at present (Bayliss) indicates that the vasodilator 

 effect is directly caused in some unknown way by afferent fibers 

 in the nerves forming the brachial and the sciatic plexus. The 

 theoretical explanations offered for this result have iDeen referred 

 to (p. 610). 



The Abdominal Organs. — The stomach and intestines receive 

 their most important supply of vasoconstrictor fibers by way of the 

 splanchnic nerves and celiac and superior mesenteric gangha. 

 These fibers emerge from the cord in the lower thoracic spinal 

 nerves, from the fifth down, and the upper lumbar nerves, and they 

 supply the whole mesenteric circulation as far as the descending 

 colon. According to some observers (Frangois-Franck and Hal- 

 lion), the mesenteric vessels receive a supply of vasodilator fibers 

 by way of the splanchnics, and it is also stated that similar fibers 

 reach this region through the vagus nerve. The pancreas has 

 been shown to receive vasoconstrictor fibers by way of the splanch- 

 nics, and the kidney, according to Bradford, receives vasodilator 

 as well as vasoconstrictor fibers from the same nerve. Most of 

 the vasomotor fibers to the kidney of the dog emerge from the 

 cord in the roots of the eleventh, twehth, and thirteenth thoracic 

 nerves, and those for the liver (Frangois-Franck and Hallion) 

 come from about the same region. The vasoconstrictors to the 

 spleen are said to leave the spinal cord chiefly in the anterior roots 

 of the sixth, seventh, and eighth thoracic nerves. 



The Genital Organs. — Both vasoconstrictor and vasodilator 

 fibers have been discovered for the external genital organs (penis, 

 scrotum, clitoris, vulva). The vasoconstrictors arise in the dog 

 from the thirteenth thoracic to the fourth lumbar nerves, pass over 

 to the sympathetic chain, and thence reach the organs either by 

 way of the hypogastric nerve and pelvic plexus or by way of the 

 sacral sympathetic ganglia and their branches to the pudic nerves. 

 The vasodilator fibers arise from the sacral spinal nerve, being the 

 best known of the sacral autonomic system. They enter the ner- 

 vus erigens and thence reach the organs by way of the pelvic 

 plexus. The especial importance of these fibers in the process of 

 erection is described in the section on the physiology of the repro- 

 ductive organs. The internal genital organs — uterus, vagina, 

 vas deferens, seminal vesicles, etc. — receive no vasomotor fibers 

 from the sacral autonomic system, — that is, from the nervi erigentes 

 — ^but do receive a supply of constrictor fibers from the sympathetic 

 system. These latter fibers emerge from the cord in the roots of 



