EESULTS OF IRRITATION. 445 



iising the microscope instead of the unaided eye, we find that 

 irritation, such as a pinch applied to t]ie web of a frog's foot, 

 causes sometimes brief contraction, succeeded by dilatation ; at 

 other times, immediate dilatation, first of the arteries, tlien of 

 the veins and lastly of the capillaries, at the site of irritation, 

 and at the same time, the velocity of the cuirent througli them 

 becomes greatly increased.* After awhile the arteries contract 

 again, the contraction beginning at a distance from the irritated 

 spot, and progressing towards it. This contraction interferes 

 with the current of blood, and its velocity in the arteries 

 beyond becomes reduced to the normal. JSText the capillaries 

 contract, but the veins still remain dilated ; the current in them 

 becomes slow, and white blood-corpuscles stick to their sides ; 

 but after a little they also contract, and the normal circulation 

 becomes completely restored. 



If a piece of caustic is applied to the web, similar changes are 

 produced : all the vessels in tlie neighbourhood, for some distance 

 around, dilate, and the blood streams through them with great 

 velocity. But here a remarkable condition makes its appear- 

 ance which was not present in the previous experiment. 

 Although the capillaries of the injured part remain dilated, and 

 the blood is streaming with unabated rapidity in the vessels all 

 around, it begins to get slower in them ; the red corpuscles seem 

 to find an impediment in their way, and accumulate in these 

 3apillaries, like the vehicles in one of the crowded streets of the 

 City during a block. The current in the arteries and veins like- 

 wise becomes slow ; the so-called lymph-spaces, which, like side- 

 wnlks, run along the interior of the arterial walls, and are usually 

 free from blood corpuscles, become invaded ; the corpuscles oscil- 

 late backwards and forwards, as if in a vain attempt to proceed, 

 and then, becoming stationary, seem to form an almost solid 

 mass. By-and-by the vessels in the neighbourhood contract 

 again, and the current in them becomes normal ; but those 

 vessels which lead directly into the cauterised part — arteries as 

 well as veins — remain permanently dilated. The stasis in the 

 capillaries extends over a wider area ; a few red corpuscles pass 

 through the walls of the capillaries, and colourless corpuscles 



* Lister, Phil. Trans., 185S, p. G45. Colinlieim, Neue Untersuchungen, 



