158 A MANUAL OF PHYSIOLOGY 



unambiguous experiment, although there are grounds on which 

 it has been surmised that the nervous system does influence the 

 ' tone ' of the whole venous tract. These grounds will be men- 

 tioned in the proper place. Meanwhile, before describing the 

 distribution of the best-known tracts of vaso-motor fibres and 

 defining the position of the vaso-motor centres, we must glance 

 at the methods by which our knowledge has been attained. 



(1) In translucent parts inspection is sufficient. Paling of the part 

 indicates constriction ; flushing, dilatation of the small vessels. 

 This method has been much used, sometimes in conjunction with 

 (2), in such parts as the balls of the toes of dogs or cats, the ear of 

 the rabbit, the conjunctiva, the mucous membrane of the mouth and 

 gums, the web of the frog, the wing of the bat, the intestines, etc. 



(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 such 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 con- 

 clusion as to local changes of resistance without knowing how the 

 general blood-pressure had varied. 



(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 

 corresponds to increase of vascular resistance in it ; increase in the 

 rate of flow implies diminished vascular resistance. Sometimes the 

 red colour of the blood issuing from a cut vein, and the visible pulse 

 in the stream, indicate with certainty that the vessels of the organ 

 have been dilated. 



