274 THE HUMAN B( >D Y. 



along the translucent organ, giving off branches which by 

 subdivision become too small to be separately visible, and the 

 whole ear has a pink color and is warm from the abundant 

 blood flowing through it. Attentive observation will show 

 also that the calibre of the main artery is not constant; at 

 somewhat irregular periods of a minute or more it dilates and 

 contracts a little. 



If the sympathetic trunk have been previously divided on 

 the other side of the neck of the animal, the ear on that side 

 will present a very different appearance. It arteries will be 

 much dilated and the whole ear fuller of blood, redder, and 

 distinctly warmer; the slow alternating variations in arterial 

 diameter also have disappeared. We get thus evidence that 

 the normal mean calibre of the artery is maintained by influ- 

 ences reaching its muscular coat through the cervical sym- 

 pathetic. Stimulation of the upper end of the cut nerve 

 confirms this opinion. It is then seen that the arteries of 

 the corresponding ear gradually contract until even the main 

 vessel can hardly be seen, and in consequence the whole ear 

 becomes pale and cold. After the stimulation is stopped the 

 arteries again slowly dilate until they have regained their full 

 paralytic size, and they usually remain permanently in that 

 condition. Sometimes they regain after some days almost 

 the size of those in the ear on the uninjured side, even when 

 the nerve has not only been cut, but the upper cervical 

 sympathetic ganglion extirpated; this seems to indicate that 

 the arterial muscle has a small automaticity of its own tending 

 to keep it in a moderate state of contraction, but it is less 

 marked than the automaticity of the myocardium. 



Quite similar phenomena can be observed in transparent 

 parts of other living animals, as in the web of a frog's foot, 

 the arteries of which dilate after section of the sciatic nerve 

 and constrict when the peripheral end of the nerve is stimu- 

 lated. In the case of other parts changes in temperature 

 may be used to detect alterations in the flow of blood. In a 

 dog or cat, for example, a sensitive thermometer placed be- 

 tween the toes indicates a rise of temperature, owing to in- 

 creased flow of warm blood through the skin, after section of 

 the chief nerve of the limb, and a fall of temperature (usu- 

 ally) during stimulation of the peripheral end of the divided 

 nerve. 



When the vaso-constrictor.nerves cut are those controlling a 



