220 THE VASCULAK MECHANISM. 



chain of splanchnic ganglia. In these ganglia the fibres undergo a remark- 

 able change. Along the anterior root and along the visceral branch they 

 are medullated fibres, but long before they reach the bloodvessels for which 

 they are destined they become non-medullated fibres ; they appear to lose 

 their medulla in the system of splanchnic ganglia. We may add that in the 

 anterior roots and along the visceral branches, white rarni communicantes, 

 these fibres are invariably of small diameter, not more than 1.8/x to 3.6 //. 



156. The course of the vaso-dilator fibres appears to be a somewhat 

 different one, some apparently accompanying the vaso-constrictor fibres, and 

 others running an independent course, though the details have as yet been 

 fully worked out in the case of only a few of the fibres. It is chiefly in the 

 nerves belonging to the cranial and sacral regions of the central nervous 

 system, whence, as we have seen, no vaso-constrictor fibres are known to issue, 

 that the course of the vaso-dilator fibres has been successfully traced. Thus 

 the vaso-dilator fibres for the submaxillary gland running in the chorda 

 tympani may be traced, as we have seen, back to the facial or seventh nerve ; 

 and the continuation of the chorda tympani along the lingual nerve to the 

 tongue contains vaso-dilator fibres for that organ ; when the lingual is stim- 

 ulated, the bloodvessels of the tongue dilate owing to the stimulation of the 

 conjoined corda tympani fibres. The ramus tympanicus of the glosso-pharyn- 

 geal nerve contains vaso-dilator fibres for the parotid gland, and it appears 

 probable that the trigeminal nerve contains vaso-dilator fibres for the eye 

 and nose and possibly for other parts. In the anterior roots of the sacral 

 nerves run vaso-dilator fibres which pass into the so-called nervi erigentes, the 

 nerves stimulation of which by leading to a widening of the arteries of the 

 penis, brings about the erection of that organ, the effect being assisted by a 

 simultaneous hindrance to the venous outflow. Though vaso-dilator fibres 

 are, as we have seen, present in the nerves of the limbs, and probably also 

 in those of the trunk, the investigation of their several paths is rendered 

 very difficult by the concomitant presence of vaso-constrictor fibres. There 

 are some reasons for thinking that the vaso-dilator fibres in these nerves pur- 

 sue a direct course from the spinal cord through the anterior spinal roots, 

 and thus afford a contrast with the constrictor fibres of the same nerves, 

 which, as we have seen, take a roundabout course, passing into the splanch- 

 nic system before they join the nerve trunk. Our information, however, 

 is too imperfect to allow any very positive statement to be made. Accept- 

 ing this view, however, we may say that while all the vaso-constrictor fibres, 

 as far as we know, come from a particular, though considerable, part of the 

 spinal cord and pass into the splanchnic system on their way to their sev- 

 eral destinations, the vaso-dilator fibres arise from all parts of the spinal 

 cord as well as from the medulla oblongata, and pursue a more or less direct 

 course to their destination. 



Further, while the vaso-dilator fibres, as they leave the central nervous 

 system, are, like the vaso-constrictor fibres, fine medullated fibres, unlike 

 the vaso-constrictors they retain their medulla for the greater part of their 

 course, and only lose it near their termination in the tissue whose blood- 

 vessels they supply. 



Lastly, while the vaso-constrictor fibres, as in the case of the cervical 

 sympathetic, of the abdominal splanchnic, and of the nerves of the skin, 

 and probably in all cases, are normally in a state of moderate activity (so 

 long as they remain in connection with the central nervous system), the 

 moderate activity maintaining that moderate constriction which we spoke 

 of above as " tone," the vaso-dilators appear to possess no such continued 

 activity. Section of vaso-constrictor fibres leads to loss of tone, diminution 

 of constriction, lasting, as we shall see, for some considerable time; but 



