234 CIRCULATION OF THE BLOOD 



sure and relaxes in consequence of a fall in pressure. According to Bayliss 

 these changes are entirely independent of the central nervous system and can 

 be demonstrated under natural conditions as well as in exsected arteries. 



10. VASODILATOR NERVES 



If the lingual nerve be stimulated, and attention be directed to the sub- 

 maxillary gland, the vessels of the gland may be seen to dilate. The veins 

 of the gland swell up, the blood flowing in them takes on a brighter color, and 

 sometimes actual pulsations appear in them. From this it follows that this 

 nerve contains fibers, the stimulation of which cause the vessels of the gland 

 to dilate. Such nerves are described as vasodilator nerves. 



Where these nerves occur unmixed with vasoconstrictor nerves, one meets 

 with no difficulty in demonstrating them. Where they are mixed with such 

 nerves for the same organ, it is necessary to adopt a special order of experi- 

 mentation, because the vasoconstrictor effect of stimulation often, if not always, 

 predominates. A strong vasodilation appears as an after-effect of the simul- 

 taneous stimulation of both kinds of nerves. The two are, therefore, not 

 strictly antagonistic, but must affect the vessels at different points, just as the 

 two kinds of cardiac nerves have different points of application in the heart 

 (V. Frey; cf. page 193). 



On the other hand, weak stimulation of the constrictor nerves is overcome 

 by stronger stimulation of the dilators. This appears, for example, in the case 

 of the submaxillary gland with the cervical sympathetic intact. Although the 

 vessels of the submaxillary gland are under the influence of the constrictor 

 fibers contained in this nerve, stimulation of the lingual produces vasodilation. 



If a nerve trunk be cut in two transversely, and the animal be allowed to 

 live, degeneration of the peripheral stump appears in a short time. If the 

 degenerating nerve be stimulated some four days after the section, vasodilation 

 is obtained (Goltz), whereas stimulation of the fresh nerve causes vasocon- 

 striction. This means that the dilator nerves retain their irritability some- 

 what longer than the constrictor nerves after connection with the central 

 nervous system has been destroyed. 



By appropriate variation of the stimulus the presence of dilator fibers can 

 be demonstrated also in freshly cut nerves. Thus it has been shown that the 

 latter are more irritable than the constrictor nerves if the stimulus is weak 

 or is applied rhythmically at a slow rate (Ostroumoff, Bowditch). 



Finally, it has been found that even if the two kinds of nerve fibers run in 

 the same peripheral nerve trunk, they make their exit by way of different 

 roots of the spinal cord, and can be separated one from the other by this 

 means (Dastre and Morat). ^ 



Other characteristics of the dilator nerves are the following: (1) the latent 

 period for their stimulation appears to be somewhat longer than that of the 

 constrictor nerves; (2) whereas the maximum effect of the constrictors is quickly 

 reached, that of the dilators takes more time; (3) the after-effect is longer. 



The Course of the Vasodilator Nerves. We have already become acquainted 

 with the vasodilator nerves which traverse the lingual nerve to the submaxillary 



