366 PHYSIOLOGY CHAP. 



widely distributed than the vaso-constrictor centres, and that 

 there is in the bulb, along with the controlling centre which 

 normally regulates the constriction of the vessels, another con-'- 

 trolling centre which regulates their relaxation, so that the slow 

 oscillations of vascular tonicity should be regarded as the effects of 

 the alternating functional predominance of one or other of the 

 two opposing bulbar vasomotor centres. The few data relating to 

 this subject may be briefly summarised. 



The rapid and infallible effect of central excitation of the 

 depressor nerves, which is antagonistic to the effect consequent on 

 central stimulation of the sciatic or other large nerve trunks, in 

 itself makes the existence of a controlling vaso-dilator centre 

 highly probable (Ludwig and Cyon). It has so far proved 

 impossible to demonstrate that it is situated in the medulla. 

 Certain experiments of Laffont (1880) are, however, of interest in 

 this connection. He observed as the effect of puncturing the 

 floor of the fourth ventricle, an active dilatation of the hepatic 

 vessels, which would therefore seem to depend not on the destruc- 

 tion, but on the stimulation of the centre. On the following day 

 stimulation of the depressors in the same animal failed to produce 

 the customary fall of blood pressure, probably in consequence of 

 paralysis due to the after-effects of the puncture in the bulbar 

 vaso-dilator centre. 



The following experimental results prove the existence of 

 vaso-dilator centres in the cord as well. In dogs that have 

 survived transverse division of the spinal cord at the level of the 

 last dorsal vertebra, mechanical stimulation of the penis is able to 

 evoke erection. After destruction of the lumbar cord this effect 

 is abolished (Goltz). In the same dog, with divided cord, the 

 central excitation of one sciatic produces active dilatation of the 

 paw on the opposite side (Goltz, Masius and Vanlair, Ustimowitsch 

 and others). When the cervical cord is excited, aortic pressure 

 falls (Johannson) ; there is dilatation of the vessels of the ears, 

 cheeks, and corresponding mucosa (Dastre and Morat); of the 

 vessels of the mesentery and intestinal walls (Vulpian); and of 

 those of the penis (Eckhard). After division of the cord at the 

 level of the first or second dorsal vertebrae, central excitation of 

 the brachial plexus causes a fall in aortic pressure (Smirnow). 



Again, the existence of cerebral vaso-dilator centres is rendered 

 probable by the fact that electrical stimulation of the cortex at 

 certain points, particularly of the parietal lobe, produces not a 

 rise but a fall of pressure in the aorta (Bochefontaine, Strieker, 

 Bechterew, and Mislawsky). These are probably the centres 

 whose activity causes the sudden blush that accompanies psychical 

 emotions. 



In 1893 Bayliss, after describing various experimental results 

 on vascular reflexes, put forward the hypothesis that " the vaso- 



