THE CIRCULATION OF THE BLOOD AND LYMPH 149 



(2) Observation of changes in the temperature of parts. This 

 method has been chiefly employed in investigating the vaso-motor 

 nerves of the limbs, the thermometer bulb being fixed between the 

 toes. In sucn peripheral parts the temperature of the blood is 

 normally less than that of the blood in the internal organs, because 

 the opportunities of cooling are greater. The effect of a freer cir- 

 culation of blood (dilatation of the arteries) is to raise the tempera- 

 ture ; of a more restricted circulation (constriction of the arteries), to 

 lower it. 



(3) Measurement of the blood-pressure. If we measure the 

 arterial blood-pressure at one point, and find that stimulation of 

 certain nerves increases it without affecting the action of the heart, 

 we can conclude that upon the whole the tone of the small vessels 

 has been increased. But we cannot tell in what region or regions 

 the increase has taken place ; nor can we tell whether it has not been 

 accompanied by diminution of tone in other tracts. 



But if we measure simultaneously the blood-pressure in the chief 

 artery and chief vein of a part such as a limb, we can tell from the 

 changes caused by section or stimulation of nerves whether, and in 

 what sense, the tone of the small vessels within this area has been 

 altered. For example, if we found that the lateral pressure in the 

 artery was diminished, while at the same time it was increased in the 

 vein, we should know that the * resistance ' between artery and vein 

 had been lessened, and that the blood now found its way more 

 readily from the artery into the vein. If, on the other hand, the 

 venous pressure was diminished, and the arterial pressure simul- 

 taneously increased, we should have to conclude that the vascular 

 resistance in the part was greater than before. If the pressure both 

 in artery and vein was increased, we could not come to any conclu- 

 sion as to local changes of resistance without knowing how the 

 general blood-pressure had varied. 



It is also sufficient to measure the blood-pressure simultaneously 

 at two points of the arterial path by which blood reaches the part, 

 provided that there is a distinct difference in the pressure at the two 

 points. The ratio of the two pressures will not be altered by any 

 general change of blood-pressure due to changes in the action of the 

 heart ; any alteration in the ratio will indicate a change in the peri- 

 pheral vascular resistance in the part beyond the more distal of the 

 two manometers. 



On this principle, Hiirthle has studied the changes in the circula 

 tion of the brain by inserting manometers into the central end of the 

 divided common carotid and the peripheral end of the internal 

 carotid. The former shows the lateral pressure in the aorta, the 

 latter that in the circle of Willis. 



(4) The measurement of the velocity of the blood in the vessels 

 of the part. This may be done by the stromuhr or dromograph, or 

 by allowing the blood to escape from a small vein and measuring the 

 outflow in a given time, or, without opening the vessels, by estimating 

 the circulation time (p. 123). When changes in the general arterial 

 pressure are eliminated, slowing of the blood-stream through a part 



