640 CIRCULATION OF BLOOD AND LYMPH. 



explained. Whether or not these fibers also pass through .the 

 sympathetic system, following the same general course as the 

 vaso constrictors, 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 been referred 

 to (p. 623). 



The Abdominal Organs. The stomach and intestines receive 

 their most important supply of vasoconstrictor fibers by way of the 

 splanchnic nerves and celiac ganglion. 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. The stomach and in- 

 testines are said to receive vasoconstrictor fibers from the vagus 

 nerve also (Lohmann). According to some observers (Franyois- 

 Franck and Hallion), 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 splanchnics, 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, twelfth, and 

 thirteenth thoracic nerves, and those for the liver (Franyois- 

 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 



