CHAP, iv.] THE VASCULAR MECHANISM. 291 



the blood on the one hand and the blood vessels with the tissues 

 on the other ; but a different state of things sets in when that 

 equilibrium is overthrown by causes leading to what is called 

 inflammation or to allied conditions. 



If an irritant, such as a drop of chloroform or a little diluted 

 oil of mustard, be applied to a small portion of a frog's web, tongue, 

 mesentery, or some other transparent tissue, the following changes 

 may be observed under the microscope ; they may be still better seen 

 in the mesentery or other transparent tissue of a mammal. The 

 first effect that is noticed is a dilation of the arteries, accompanied 

 by a quickening of the stream. The irritant, probably by a direct 

 action on the muscular fibres of the arteries, has led to a re- 

 laxation of the muscular coat, and hence to a widening ; and we 

 have already, 105, explained how such a widening in a small 

 artery may lead to a temporary quickening of the stream. In 

 consequence of the greater How through the arteries, the capillaries 

 become filled with corpuscles, and many passages, previously 

 invisible or nearly so on account of their containing no corpuscles, 

 now come into view. The veins at the same time appear enlarged 

 and full. If the stimulus be very slight, this may all pass away, 

 the arteries gaining their normal constriction, and the capillaries 

 and veins returning to their normal condition ; in other words, the 

 effect of the stimulus in such a case is simply a temporary blush. 

 Unless, however, the chloroform or mustard be applied with especial 

 care, the effects are much more profound, and a series of remarkable 

 changes set in. 



In the normal circulation, as we have just said, white corpuscles 

 may be seen in the peripheral, plasmatic zone, but they are scanty 

 in number, and each one, after staying for a little time in one spot, 

 suddenly gets free, sometimes almost by a jerk as it were, and then 

 rolls on for a greater or less distance. In the area now under 

 consideration a large number of white corpuscles soon gather in 

 the peripheral zones, especially of the veins and venous capillaries 

 (that is of the larger capillaries which are joining to form veins), 

 but also, of the other capillaries, and, to a less extent, of the arteries ; 

 and this takes place although the vessels still remain dilated and the 

 stream still continues rapid, though not so rapid as at first. Each 

 white corpuscle appears to exhibit a greater tendency to stick to 

 the sides of the vessels, arid though driven away from the arteries 

 by the stronger arterial stream, becomes lodged so to speak in the 

 veins. Since new white corpuscles are continually being brought 

 by the blood stream on to the scene, the number of them in the 

 peripheral zones of the veins increases more and more, and this 

 may go on until the inner surface of the veins and venous 

 capillaries appears to be lined with a layer of white corpuscles. 

 The small capillaries too contain more white corpuscles than 

 usual, and even in the arteries these are abundant, though not 

 forming the distinct layer seen in the veins. The white cor- 



