THE VASCULAR MECHANISM. 225 



effect of section or stimulation of these nerves on the calibre of the 

 arterioles has been ascertained, not by direct ocular observation, but by 

 determining the amount of .blood flowing through the organ in a given 

 time. The volume of blood (V) flowing through an organ in a given 

 time varies directly with the mean arterial pressure (P) and inversely 



with the resistance (R) in its arterioles, and is represented by the 



p 

 formula Voc -. Hence the rate of blood flow through a small organ 



R 



such as the kidney may be altered in one of two ways. On the one 

 hand, in the absence of any active change in its arterioles, a rise of 

 the general arterial blood pressure will force more blood through the 

 arterioles of the kidney. On the other hand, if the general pressure 

 remains constant, dilatation of the renal arterioles will lead to an 

 increased rate of blood flow through the kidneys by lessening the 

 resistance to the flow of blood. In experiments on the rate of blood 

 flow through an organ it is necessary, therefore, to record both the 

 rate of flow and the general arterial pressure, in order to ascertain 

 whether the alterations in flow are due to local changes in the arterioles, 

 or to changes in the general arterial pressure, or possibly to a com- 

 bination of these factors. 



The amount of blood flowing from an organ in a given time may 

 be directly measured by allowing the blood escaping by the veins to 

 pass along a graduated tube. Thus if 2 c.c. of blood flow into the 

 tube in 4 seconds, the rate of flow is 30 c.c. per minute. This method is 

 very useful in the case of small organs such as the kidney or sub- 

 maxillary gland. 



Another method is to record the variations in volume of the organ. 

 These variations depend almost entirely upon the amount of blood 

 present in the organ at any moment, and this will alter with the degree 

 of dilatation or constriction of its blood-vessels. The organ is placed in 

 an air-tight box or instrument known as a plethysmograph, provided 

 with a small opening which is connected with a tambour and a record- 

 ing lever. When the organ expands, the air in the box is driven along 

 the tube into the tambour, thereby raising the lever ; shrinkage of the 

 organ has the opposite effect. The form of plethysmograph varies 

 with the shape of the organ which is being studied. The one generally 

 used for the kidney, and known as an oncometer, is made of vulcanite 

 with a glass lid ; in one side is a groove through which the renal 

 vessels and nerves can pass (fig. 87). The box is made air-tight by 

 filling the interstices with vaseline. A glass tube passing through its 

 wall is connected with a piston recorder (p. 199). Fig. 88 represents 

 a record of the kidney volume thus obtained, simultaneously with a 



15 



