698 



PRINCIPLES OF GENERAL PHYSIOLOGY 



of the fact that no dilators, other than the dorsal root fibres, can be detected, 

 so that these must be excited from the centre in an opposite direction to that 

 which would be considered to be the normal one. Fig. 244 is from an experiment 

 of my own, also showing this fact. 



Martin and Mendenhall (1915) have shown that there is vaso dilatation in the 

 nasal mucous membrane when the depressor is stimulated, although the vaso- 

 constrictors were cut. 



In Fig. 245 a diagram of the connections of the two vasomotor centres is 

 given ; this may assist in following the somewhat complex state of affairs. 



Sherrington (1913, 2, p. 93) interprets these phenomena as being allied to the postural 

 tonus of skeletal muscle. The automatic tonus of the arterial wall is postural as regards the 

 contained blood. As it is only under special conditions that flexor tonus can be obtained, so 

 it appears that tonus of the vaso-dilator centre is not usually to be detected. 



FIG. 248. VASO-CON- 

 STRICTION BY INHI- 

 BITION OF DILATOR 

 TONE. Ear of 

 rabbit. Sympathetic 

 cut. 



Upper curve, volume of ear. 



Lower curve, blood pressure. 



At the signal, the ot-Mtnil 

 end of the median nerve 

 was stimulated. There 

 was only a slight rise of 

 blood pressure, because 

 the animal had been 

 eviscerated. Constriction 

 of the ear is shown, pre- 

 ceded by slight dilatation. 



(Bayliss, 1908, 2, 

 Fig. 22.) 



FIG. 249. INHIBITION OK 

 DILATOR TONE CONVER- 

 TED BY STRYCHNINE INTO 

 EXCITATION OF DILATORS. 



Same experiment as in Fig. 248, 

 but after the injection of 

 strychnine. 



Stimulation of the median m-rvc 

 caused ililntati'ni of the ear, 

 accompanied by a slight fall 

 of arterial prf*Mirc. 



(Bayliss, 1908, 2, 

 Fig. 23.) 



Loven Reflexes. An interesting form of local vascular reflex was first 

 described by Loven (1866). When the central end of the great auricular nerve, 

 the sensory nerve of the ear in the rabbit, was stimulated, it was noticed that, 

 although vaso-constriction was produced in other organs, in the ear itself vaso- 

 dilatation occurred. A similar effect was obtained in the leg. Fig. 246 gives 

 two tracings of this latter effect. It appears, thus, that an active organ may 

 bring about a better blood flow to itself, not only by raising the general blood 

 pressure, but by a local vaso-dilatation. I showed that double reciprocal 

 innervation holds also in these reflexes (1902, 3, p. 292, and 1908, 2, pp. 351-353) ; 

 since it appears that the dilators to the ear are probably antidromic in nature- 

 (see Bayliss, 1906, 3, p. 330), we have evidence of another case of reflex 

 stimulation of dorsal root sensory fibres. 



