THE CIRCULATION OF THE BLOOD 



701 



produce dilatation in the gland vessels. The conclusion seems justified that the 

 failure is on account of the absence of secretion. Some observers appear to have 

 obtained, normally, vaso-dilatation from the cervical sympathetic, not preceded 

 by constriction. This has not been my 

 experience. I find constriction at first, 

 followed by dilatation, as the secretion 

 becomes more copious. 



Reaction to Changes of Pressure. I 

 had observed (1902, 2), that if the 

 general blood pressure is suddenly 

 raised while a plethysmographic trac- 

 ing is being taken of an organ, that the 

 first passive expansion of the organ is 

 followed by a considerable contraction. 

 Since it was known that smooth muscle 

 in various situations responds to stretch- 

 ing by contraction, I interpreted the 

 effect as being due to a similar reaction 

 on the part of the muscular wall of the 

 arterioles. The fact that stimulation 

 of the splanchnic nerves, or asphyxia, 

 excites the flow of adrenaline into 

 the blood current was not known at 

 the time, and Anrep (1912, 2) has 

 shown that the methods used by me 

 to cause a rise of blood pressure were 

 such as to cause this flow, and that the 

 results are sufficiently accounted for by 

 it. Of course, the reaction to stretch- 

 ing may also take place, but my experi- 

 ments did not prove it. Anrep was 

 unable to confirm the result which I 

 had obtained by raising the pressure 

 inside an excised piece of artery, but 

 my recollection of this experiment is 

 so clear that I am unable to believe 

 that the particular piece of artery used 

 then did not really contract. If I 

 understand Anrep's description of his 

 method correctly, it does not seem pos- 

 sible for the artery to contract when it 

 was distended by pushing in the piston 

 of a syringe. In the same paper I also 

 described the opposite reaction of 

 blood vessels to fall of pressure, in 

 which it appeared that dilatation took 

 place in response to diminution of 

 tension. The effect of adrenaline is 

 excluded here, and Anrep explains the 

 result as due to asphyxial metabolic 

 products. I do not feel quite satisfied 

 with this explanation, but I was 

 unfortunately unable to see Anrep's 

 experiments. No doubt the pro- 

 longed stoppage of circulation in his 

 experiments was sufficient to afford such metabolites, but if Fig. 250 be con- 

 sulted, it will be seen that a compression of the abdominal aorta for eight seconds 

 produces a subsequent dilatation of nearly as great a degree as one of 

 twenty seconds, and it is difficult to believe that deprivation of blood flow for 



Fiu. 250. LOCAL REACTION OF BLOOD VESSELS 



TO FALL OF INTERNAL PRESSURE. 



Upper curve, volume of hind leg of dog, constrictors and 

 dilators cut. 



Lower curve, pressure in femoral artery. Zero at level 

 of upper signal. 



Time in ten-second intervals. 



The abdominal aorta was compressed twice, the first time 

 only being marked by the signal. The blood pressure 

 curve shows the actual duration of the fall of pressure 

 in the limb vessels in both cases. 



The first fall in volume, due to deprivation of blood, is 

 followed by a large dilatation. The magnitude of 

 this dilatation does not seem to have any relation to 

 the duration of the anaemia, and it is difficult to under- 

 stand how an anaemia of only eight seconds' duration 

 in a curarised animal under artificial respiration could 

 produce sufficient asphyxial metabolites to cause so 

 large an effect. 



The dilatation is followed by a constriction, which may 

 possibly be a contractile response to the sudden in- 

 rush of blood into the dilated vessels. 



(Bayliss, 1902, 2, Fig. 7.) 



