318 COURSE OF VASO-DILATOR FIBRES. [BOOK i. 



white rami communicantes, these fibres are invariably of small 

 diameter, not more than 1'8 //, to 3'6 //,. 



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

 wholly different one, though the details have as yet been fully 

 worked out in the case of few of the fibres only. 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-con- 

 strictor fibres are known to issue, that the course of the vaso-dilator 

 fibres has been successfully traced. Thus the vaso-dilator fibres for 

 the sub-maxillary 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 stimulated, the blood vessels of the tongue 

 dilate owing to the stimulation of the conjoined chorda tympani 

 fibres. The ramus tympanicus of the glossopharyngeal 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 second and third sacral nerves run vaso-dilator fibres 

 which pass into the so-called nervi erigentes, the nerves, stimula- 

 tion 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 

 pursue 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 splanchnic system, before they join the 

 nerve trunk. Our information however is too imperfect to allow 

 any very positive statement to be made. Accepting 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 several 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 



