viii BLOOD-STREAM: MOVEMENT IX VESSELS 255 



the small arteries, is richly provided. The increased resistance to 

 the passage of the blood, provoked by the augmentation of vascular 

 tone, depends both on the degree of this augmentation, and on its 

 extension over a more or less extensive vascular area. 



In like manner, the diminution of vascular resistance must, under 

 physiological conditions, depend upon a more or less pronounced 

 or diffuse paralysis or diminution of tone in the vessels. In the 

 next chapter we shall study the physiological adaptations for 

 regulating cardiac and vascular activity, and the processes by 

 which the central and peripheral variations of the circulation tend 

 to become compensated. 



In order to determine the dependence of blood pressure on the 

 mass of blood contained in the system, it is obvious that the 

 effects on blood pressure of transfusion and bleeding must be 

 considered. This subject was studied by Tappeiner, and in an 

 exhaustive manner by Worin-Muller in 1873, in Ludwig's labora- 

 tory. The results of the experiments show that in the dog during 

 the increments in blood pressure produced by successive transfusions 

 of homogeneous defibrinated blood, or the corresponding decrement 

 produced by successive haemorrhages, the physiological limits vary 

 very little, far less than would be expected from the amount of 

 blood added to or taken from the system. Further, such rise or 

 fall in arterial pressure is of very brief duration, and therefore can 

 only be influenced to a minimal extent by increase or diminution 

 in transudations, and urinary secretions, through the capillaries. 

 There must, therefore, be some compensatory mechanism, which 

 tends rapidly to restore blood pressure to the normal, by producing 

 a dilatation or constriction of the small arteries and capillaries, 

 which adapts them even to very considerable alterations in the 

 blood content. 



Pawlow's researches (1878), which confirm Worm - Miiller's 

 observations by another method, must also be noted. When a 

 dog was fed on dry bread or meat he found that blood pressure 

 fell 10 mm. Hg in an artery of the thigh, owing to the 

 dilatation of the intestinal vessels and digestive secretions. On 

 giving the same dog a large quantity of broth he found no rise of 

 blood pressure. There must, therefore, be some mechanism which 

 promptly reduces an increase or decrease in the amount of fluid 

 contained in the body to its normal limits. 



1'awlow further showed on dogs that, during complete rest and 

 sensory inactivity, blood pressure from day to day does not alter. 

 On the other hand, it increases slightly after meals, arid sinks 

 slightly in the morning. It regularly becomes lower after a warm 

 bath. The abrupt upward or downward changes in arterial 

 pressure are due to disturbances of vascular inner vation (infra, 

 Chapter X.). 



The results of a series of experiments which Colombo carried 



