THE NERVOUS REGULATION OF THE BLOODVESSELS 179 



Finally, adrenalin (epinephrin) causes constriction of rings of 

 ' surviving ' veins just as of artery rings, although in correspondence 

 with the smaller amount of muscular tissue in the former the con- 

 traction is not so strong. As adrenalin is assumed to act only upon 

 muscle supplied by sympathetic nerve-fibres (p. 638), this would 

 seem to indicate the existence of such a supply for veins. The 

 question is an important one in connection with the regulation or 

 the filling, and therefore of the discharge, of the heart (Henderson), 

 but the experimental data are as yet too meagre to justify further 

 discussion of the matter here. 



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

 pass out as fine medullated fibres (1-8 to 3-6 p. 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 sym- 

 pathetic ganglia, where, or in more distal ganglia such as the inferior 

 mesenteric, they lose their medulla, and their axis-cylinder processes 

 (p. 822) 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 bloodvessels. 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 fibies 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 membrane 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 wayof 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 dis- 

 tributed 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 statement has been 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). 



