174 THE CIRCULATION OF THE BLOOD AND LYMPH 



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 know- 

 ledge 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 circulation of blood (dilatation of the 

 arteries) is to raise the temperature ; 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 simultaneously 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 conclusion as to local changes of resistance with- 

 out 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. 135). 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. 



(5) Alterations in the volume of an organ or limb are often taken as 

 indications of changes in the calibre of the small vessels in it. We 

 have already seen how these alterations are recorded by means of a 

 plethysmo graph (p. 128). The brain is enclosed in the skull as in a 

 natural plethysmograph, and changes in its volume may be registered 

 by connecting a recording apparatus with a trephine hole. 



