THE CIRCULATION OF THE BLOOD 687 



contraction of the arteries at the latter part of each systolic, wave, so that the blood is 

 forced onwards by it. On the assumption that the rate of the current of blood through 

 a certain length of artery is proportional to the difference of pressure between the two 

 ends, it was found, under certain conditions, that the rate of the current was greater towards 

 the end of the systolic part of the pulse wave than corresponded to the difference of pressure 

 at this time. It is also stated that the amplitude of the pulse wave, instead of decreasing 

 towards the periphery, as it does in the dead animal, is increased in the living animal, 

 especially when the blood vessels are constricted. The interpretation of the facts is difficult, 

 and further investigation is required. There is, of course, the possibility that the muscle 

 of the arterial wall may respond to distension by a contraction, as that of the earthworm 

 and the frog's stomach does (page 436). This idea is supported by some observations by 

 Carl Tigerstedt (1913), who found an electrical change in the carotid artery with each heart 

 beat. The direction of the deflection was such as to imply that the electrode nearest the 

 heart became negative before the more distant one. 



KEY TO FIG. 235. 



1. M. Grehant. 2, M. Dumontpallier. 3, M. Malassez. 4, M. Paul Bert. 5, M. D' Arson val 



6, M. Claude Bernard. 7, M. Dastre. 



THE PERIPHERAL RESISTANCE 



The nature of this, as conditioned by the internal friction of the blood, has been 

 explained above (pages 241-242). The blood of the dog has a viscosity about 

 five times that of water. How far changes in this property occur in physiological 

 conditions and their effect on the blood pressure have not received much attention. 

 According to Burton-Opitz (1911), the viscosity of normal blood is much higher 

 than that of defibrinated blood. Deep narcosis with ether increases the viscosity, 

 which falls again as the narcosis is diminished. Carbon dioxide also increases 

 the viscosity; hence venous blood, in addition to the effect of loss of water, 

 has a slightly higher viscosity than arterial blood. The viscosity, also, as would 

 be expected, increases with the number of corpuscles per unit volume ; and that 

 of laked blood is less than that of the same blood before laking. Of course, 

 dilution of the blood, as happens after loss of blood, has a considerable effect in 

 reducing the internal friction. 



The general effect of the existence of the peripheral resistance is to enable 

 the heart to produce a high arterial pressure, with the advantages as regards 

 regulation of blood supply to organs following therefrom, as already pointed out. 

 It is scarcety necessary to say that the peripheral resistance must not be stated to 

 be the cause of the blood pressure, which is due to the energy produced by the 

 muscular contractions of the heart. 



THE VOLUME OF THE BLOOD 



It has been shown by Dreyer and Ray (1910) that the volume of the blood 

 in mammals is satisfactorily given by the formula 



B-W"x* 



k 

 where B is the blood volume in cubic centimetres, W the weight of the animal 



