.THE CIRCULATION OF THE BLOOD 383 



tion of the sympathetic chain and by their union subsequently assist in 

 the formation of the splanchnic nerves the terminal fibers of which 

 arborize around the cells of the collateral ganglia, viz. : the semilunar, 

 superior mesenteric renal, etc. From these various ganglia an elaborate 

 network of non-medullated fibers passes to the blood-vessels of the 

 stomach, intestines, and other viscera. The great splanchnic nerve is 

 one of the most important vaso-constrictor trunks of the body, on ac- 

 count of the large vascular area it controls (Fig. 176 C). 

 The existence, course, distribution, and functions of the vaso-constrictor 

 nerves have been determined by a variety of methods, physiologic and ana- 

 tomic. Stimulation of the nerve trunks under appropriate conditions 

 gives rise to a contraction, division to a dilatation of the blood-vessels. 

 The physiologic continuity of the pre-ganglionic fibers with the nerve-cells 

 of the sympathetic ganglia has been shown by the intra-vascular injection 

 or the local application of nicotin. This agent, as shown by Langley, has 

 a selective action on the arborizations of the pre-ganglionic fibers, and when 

 given in sufficient doses suspends their conductivity; hence stimulation of 

 pre-ganglionic fibers is without effect, though stimulation of the post- 

 ganglionic fibers is followed by the usual contraction. 



The following facts will serve as illustrations of the functions of vaso- 

 constrictor nerves. Division of the great splanchnic is followed by a marked 

 dilatation of the blood-vessels of the intestinal tract and a decided fall in 

 blood-pressure; stimulation of the peripheral end by their contraction and a 

 marked rise in blood-pressure. Division of the cervical cord of the sym- 

 pathetic is followed by dilatation of the blood-vessels of the side of the head; 

 stimulation of the peripheral end by their contraction. 



The Vaso-dilatator Nerves. The vaso-dilatator nerve fibers may be 

 divided for convenience into two groups, viz., (i) those contained in the 

 trunks of some of the cranial nerves, e.g.j the nerve of Wrisberg and the 

 glosso-pharyngeal and those contained in the trunks of some of the sacral 

 nerves; (2) those contained in the trunks of the spinal nerves distributed 

 to the skin of the fore and hind limbs and to the body walls. 



i. The cranial vaso-dilatator nerve fibers also consist of two consecu- 

 tively arranged neurons, a pre- and a post-ganglionic. The pre-ganglionic 

 have their origin in nerve-cells in the gray matter of the medulla oblongata. 

 From these cells they pass out in the trunk of the pars intermedia or nerve 

 of Wrisberg and in the trunk of the glosso-pharyngeal nerve. Those 

 fibers which are contained in the nerve of Wrisberg, enter, after a short 

 course, the trunk of the facial nerve and through its branches the great 

 petrosal and the chorda tympani, are ultimately distributed as pre-gan- 

 glionic fibers to the spheno-palatine and submaxillary ganglia respec- 

 tively. From the spheno-palatine ganglion cells post-ganglionic fibers are 

 distributed to the blood-vessels of the mucous membrane of the nasal 

 chambers posteriorly, and to adjacent regions. From the submaxillary 

 ganglion post-ganglionic fibers pass to the blood-vessels of the submax- 

 illary and sublingual glands. 



The vaso-dilatator fibers that are contained in the glosso-pharyngeal 

 nerve pass through the tympanic plexus by way of Jacobson's nerve to the 

 otic ganglion, around the cells of which their end branches arborize; from 



