THE CIRCULATION OF THE BLOOD AND LYMPH 165 



Course of the Vaso-motor Nerves. In the dog the vaso-constrictors 

 pass out as fine medullated fibres (r8 to 3- 6 yu, in diameter) in the 

 anterior roots of the second dorsal to about the second lumbar 

 nerves. They proceed by the white rami communicantes to the 

 lateral sympathetic ganglia, where, or in more distal ganglia such 

 as the inferior mesenteric, they lose their medulla, and their axis- 

 cylinder processes (Chap. XII.) break up into fibrils that come into 

 close relation with the nerve-cells of the ganglia. These ganglion- 

 cells in their turn send off axis-cylinder processes, which, enveloped 

 by a neurilemma, pass as non-medullated fibres by various routes 

 to their final destination, the unstriped muscular fibres of the blood- 

 vessels. Their course to the head has been already described. To 

 the limbs they are distributed in the great nerves (brachial plexus, 

 sciatic, etc.), which they reach from the sympathetic ganglia by 

 the grey rami communicantes. 



The outflow of vaso-dilator fibres is not restricted to the same 

 portion of the cord from which the outflow of constrictor fibres 

 takes place. Their existence is indeed most easily demonstrated in 

 nerves springing from those regions of the cerebro-spinal axis from 

 which vaso-constrictor fibres do not arise, and where, therefore, we 

 have not to contend with the difficulty of interpreting mixed effects. 

 Vaso-dilators for the external generative organs and the mucous mem- 

 brane of the lower end of the rectum pass out as small medullated 

 fibres of the anterior roots of certain of the sacral nerves (mainly the 

 second and third in the cat) into the pelvic nerve (nervus erigens). 

 They end in relation with ganglion-cells in the neighbourhood of the 

 organs which they supply. The seventh and ninth cranial nerves 

 carry vaso-dilator fibres which are distributed by way of the lingual 

 and other branches of the fifth to the salivary glands, the tongue, 

 the mucous membrane of the floor of the mouth, and part of the soft 

 palate. Those in the lingual, passing through the chorda tympani,end 

 in ganglion-cells near the submaxillary and sublingual glands, and the 

 axons of these cells continue the path to the vessels of the glands. 

 It is supposed that the vaso-dilators distributed in other branches 

 of the fifth also have ganglion-cells on their course. In fact, there 

 is good evidence that every efferent vaso-motor fibre is interrupted 

 by one, and only by one, ganglion-cell between the cord and the 

 bloodvessels. The remarkable statement has been recently made 

 that for certain regions of the body, especially the skin of the limbs, 

 the vaso-dilator nerves are contained, not in the anterior, but in the 

 posterior roots. And these, it is claimed, are not aberrant efferent fibres 

 which have strayed in the course of development into the wrong roots, 

 but true posterior root-fibres whose cells of origin lie in the spinal 

 ganglia, and which conduct efferent vaso-dilator impulses in the wrong 

 direction, so to speak, from the cord to the periphery' antidromic ' 

 impulses (Bayliss). But the question is still under discussion. 



Effect of Nicotine on Nerve-cells. A method which has been 

 found most fruitful in studying the relations of sympathetic ganglion- 

 cells to the vaso-motor fibres, as well as to the pilo-motor* and 

 secretory fibres which in certain situations are so intricately mingled 

 with them, must here be mentioned. It depends upon the fact that 

 when a suitable dose of nicotine (10 milligrammes in a cat) is injected 

 into a vein, or a solution is painted on a ganglion with a brush, the 

 passage of nerve-impulses through the ganglion is blocked for a time 

 (Langley). The 1 nerve-fibres peripheral to the ganglion are not 



* Pilo-motor nerves supply the smooth arrector pili muscles, whose 

 contraction causes the hair to '^stand on end.' 



