VASOMOTOR REFLEXES. 167 



dilate, or primarily dilate. 1 On stimulation of the central ends of the 

 posterior roots of the eleventh to the thirteenth thoracic nerves, the 

 kidney expands. 2 Excitation of the entrance to the glottis flushes the 

 mylo-hyoid muscle of the frog. 3 



The sensory excitation of one side of the body frequently evokes vaso- 

 motor changes on both sides. Thus a drop of mustard, introduced into 

 the lymph sac of the frog's leg, causes congestion of the leg on the same 

 side, and contraction of the vessels on the opposite side. 4 Compression 

 of the median artery of the rabbit's ear with the finger-nail causes 

 dilatation of that ear and contraction of the opposite ear. Stimulation 

 of the central end of the posterior auricular or sciatic nerve on one side, 

 may cause vasomotor changes in both ears or both hind-limbs. Excita- 

 tion of the mucous membrane of the nose may cause vascular dilatation 

 all over the face, but more on the same side as the stimulus. 5 



It is a simple experiment to verify the fact that the vessels of the 

 one arm contract, if the other be placed in hot water. Similarly, by 

 electrical excitation of the arm, vasomotor changes may be evoked in 

 the opposite arm. 6 The latent time between the excitation and the 

 reflex results usually occupies several seconds. The place and the kind 

 of stimulus influence the reflexes obtained. For example, greater 

 changes of general arterial pressure are evoked by excitation of the 

 posterior roots of the thoracic nerves than by stimulation of the peri- 

 pheral nerves. 7 The afferent fibres from the viscera produce the most 

 marked vasomotor reflexes. 8 



In lightly curarised rabbits, a rise of arterial pressure can be evoked 

 by blowing on or stroking the skin, while more violent stimulation by 

 chemical irritants or heat scarcely produces any effect. 



Although the usual result of stimulating a sensory nerve is a reflex 

 rise of arterial pressure, a fall may occur. There is one afferent nerve, 

 and one only, which constantly produces this result, and that is the 

 depressor nerve (see p. 59). Efforts have been directed towards finding 

 depressor fibres in the common nerve-trunks. General reflex dilator 

 effects are often obtained on excitation of a sensory nerve, when the 

 vasomotor tone is weakened after prolonged experiment, 9 or during 

 administration of chloral and chloroform. 



Hunt 10 lias succeeded in demonstrating afferent depressor fibres in the 

 central end of the sciatic nerve by the following methods : — 



1. On regeneration of the nerve after section, the depressor fibres are the 

 first to recover their function. Thus, a rise of arterial tension results from 

 stimulation of the central end of a normal sciatic, a fall of tension from excita- 

 tion of the regenerating sciatic. 2. Excitation with a feeble strength of 

 current is more likely to produce a fall, while a strong current produces a 

 rise of pressure, 3. After cooling or long exposure of the sciatic nerve, 

 depressor effects may he evoked. 



Hunt classifies the afferent nerves in reference to their reflex vasomotor 



1 Loven, Per. d. k. Sachs. Gcsellsch. d. Wisscnsch. , math.-jihys. CI., Leipzig, 1866, S. 92. 



2 Bradford, Joum. Physiol., Cambridge and London, 1889, vol. x. p. 396. 



3 Gaskell, Joum. Anat. and Physiol., London, 1877, vol. xi. p. 742. 



4 Ranvier, Compt. rend. Acad. d. sc, Paris, tome cxiv. p. 629. 



; ' Francois-Fianek, Arch, dc jyliysiol. norm, etpath., Paris, 1889, p. 550. 



6 Maragliano and Lussano, Arch. ital. dc biol., Turin, 1889, vol. xi. p. 246. 



7 Bradford, Joum. Physiol., Cambridge and London, 1889, vol. x. p. 400. 



8 Heidenhain and Griitzner, Arch. f. d. ges. Physiol., Bonn, 1878, Bd. xvi. S. 54. 



9 Cyon, ibid., 1874, Bd. viii. S. 334. 



i0 Joum. Physiol., Cambridge and London, 1895, vol. xviii. p. 381. 



