82 ELEMENTARY THYSIOLOGY less. 



merely the external evidences of changes taking place at 

 the same time in the minute blood-vessels and circulation 

 of tlie part affected, and since these changes throw an 

 interesting light on the relations ordinarily existing 

 between the walls of the blood-vessels and the neighbour- 

 ing blood, they are wortliy of a short consideration. 



If when the web of a frog's foot, or other suitably 

 transparent part of an animal, is adjusted for observation 

 under the microscope, some irritant be applied to it such 

 as a trace of mustard,' the following events may be readily 

 observed. The minute arteries dilate, the blood flows 

 faster, and the increased quantity of blood forced through 

 the capillaries distends them so that they, as well as the 

 smallest veins, appear to be similarly dilated. This 

 accounts for the initial greater redness and waniitli of an 

 inflamed part. Very soon the colourless corpuscles are 

 seen to be collecting in large numbers in the clear layer 

 of fluid next to the walls of the capillaries and veinlets, 

 and seem to adlicre more firmly than usual to the walls of 

 these vessels. Further, blood "platelets" (see p. 103), 

 not previously visible, begin to collect also with and 

 among the white corpuscles. Following upon this the 

 stream of blood begins to flow more slowly although the 

 blood-vessels are still widely dilated. And now a very 

 striking phenomenon takes place. The white corpuscles 

 make their way by amoeboid movements thi'ough the thin 

 walls of the capillaries and collect outside them in the 

 spaces in the neighbouring tissue. At the same time 

 that the corpuscles are in this way " migrating," a con- 

 siderable (quantity of the fluid part of the blood also 

 passes out through the walls of the blood-vessels into the 

 adjacent tissue. This accounts for the cliaracteristic 

 swelling of an inflamed part. If the action of the irritant 

 is continued, more and more white corpuscles collect in 

 the vessels, the blood-flow becomes slower and slower, 



1 Used similarly as an irritaut in the form of the ordinary domestic 

 mustard poultice. 



