234 THE CIRCULATION OP THE BLOOD 



roots. Tho thoracic fibers pass down the sympathetic chain, which they leave by the 

 great splanchnic nerves. The lumbar fibers form the lesser or abdominal splanchnic 

 nerves. As preganglionic fibers, therefore, these fibers are carried by the greater and 

 lesser splanchnic nerves into the abdomen, where the former comes into close relation- 

 ship with the suprarenal gland's, giving off a branch to the suprarenal ganglion. The 

 main course of the nerve is continued on to the solar plexus, in the various ganglia of 

 which most of the preganglionic fibers end by synapsis, the postganglionic fibers then 

 proceeding along the blood vessels to the vessels of the abdominal viscera. (See also 

 page S70). 



Vasodilator fibers have a more varied origin than vasoconstrictor, and 

 they run an entirely different course. Vasodilator impulses may he 

 transmitted by fibers arising from practically any level of the cerebro- 

 spinal axis, not only by the motor roots, but by the sensory as well. 

 Thus, they pass out of the spinal cord in the posterior sacral roots to 

 enter the nerves of the hind limbs, as has been demonstrated by observ- 

 ing an increase in the volume of the curarized limb during electrical 

 stimulation of the exposed rootlets. The apparent inconsistency of these 

 observations with the well-known law concerning the direction of the 

 impulses contained in the posterior spinal roots is explained by assum- 

 ing that the dilator impulses are transmitted along the ordinary sensory 

 fibers, since there are no efferent fibers in these roots. They are impul- 

 ses which go against the ordinary stream (antidromic). In support of 

 this explanation it is of importance to note that at their termination 

 near the skin many sensory fibers split into several branches, some of 

 which run to blood vessels, and others to receptor organs (page 797). 

 Stimulation of the latter branches may cause dilatation of the local blood 

 vessels nearby, indicating that impulses must be transmitted up to the 

 point at which the branching occurs and then down the vascular branch, 

 this result being obtained even after the main trunk of the nerve has 

 been cut above the division. 



For the blood vessels of the anterior extremity, the vasodilator impulses are similarly 

 transmitted through the posterior spinal roots of the lower cervical region of the spinal 

 cord. The vasodilator fibers to the abdominal viscera are transmitted with the splanchnic 

 nerves, but they may also be derived from the posterior spinal roots, for it has been 

 found that stimulation of posterior roots in the splanchnic area causes dilatation in the 

 intestine (Bayliss). Vasodilator fibers are also contained in the cranial nerves, par- 

 ticularly the seventh and the ninth, being distributed in the former nerve to the an- 

 terior portion of the tongue and the salivary glands, and in the latter to the posterior 

 portion of the tongue and the mucous membrane of the floor of the mouth. The vaso- 

 dilator fibers for the mucous membrane of the inside of the cheeks and nares have their 

 course in the cervical sympathetic, being distributed to the buccofacial region in the 

 branches of the fifth cranial nerve. 



There is evidence to show that the vasodilator libers, like the vasoconstrictor, become 

 connected by synapsis with nerve cells somewhere in their course. In the case of the 

 vasodilator fibers in the chorda tympani and nervi erigentes, such cell stations have 

 been clearh demonstrated in the liilus of the submaxillary gland in the former nerve 



