THE PERIPHERAL RESISTANCE 83 



effect upon the heart-muscle a decrease in the output per 

 beat (p. 70) and, reflexly through the vagus, slowing of 

 the heart. The peripheral resistance may therefore be said 

 to influence the heart in two ways, backwards through the 

 arteries and forwards through the veins. 



THE PERIPHERAL RESISTANCE 



As already stated, the peripheral resistance is the resultant 

 of two factors, the viscosity of the blood and the cahbre 

 of the arterioles. The viscosity of the blood is due partly 

 to the plasma, partly to the corpuscles. It decreases with 

 rise of temperature and increases with the COg content. 

 In the present state of our knowledge it is impossible to 

 assess what effect such variations will have upon the 

 resistance under physiological conditions. 



Concerning variations in the calibre of the blood-vessels, 

 our knowledge is much more extensive. In thin tissues, 

 like the frog's mesentery or the rabbit's ear, such variation 

 can be directly observed. In organs such as the intestines, 

 kidney or limbs, changes in the capacity of the blood- 

 vessels are inferred from changes in the volume of the 

 whole organ. The organ is inserted into a plethysmograph, 

 which consists of a box opening equatorially. In the box 

 are two holes. One is for the blood-vessels. This is made 

 water-tight by packing with vasehne. The other hole is 

 to convey oil with which the organ is surrounded to a 

 tambour connected with recording apparatus. When the 

 organ expands oil is driven out of the box and raises the 

 recording lever. A special form of plethysmograph used 

 for the kidney is called an oncometer, and for the heart a 

 cardiometer. 



Another method apphcable to small tissues is to 

 measure the venous outflow. This has the disadvantage 

 of entailing a loss of blood. 



The factors controlUng the calibre of the blood-vessels 

 are two — nervous and chemical. 



