CHAP, iv.] THE VASCULAR MECHANISM. 295 



stream of defibrinated blood is artificially driven through a 

 perfectly fresh excised organ, such as the kidney, it is found that 

 the resistance to the flow of blood through the organ, measured, 

 for instance, by the amount of outflow in relation to the pressure 

 exerted, varies considerably owing to changes taking place in the 

 organ, and may be increased by increasing the venous character, 

 and diminished by increasing the arterial character of the blood. 

 Remarkable changes in the resistance are also brought about by 

 the addition of small quantities of certain drugs such as chloral, 

 atropin &c. to the blood. 



These changes have been attributed to the altered blood acting 

 on the walls of the vessels, inducing, for instance, constriction or 

 widening of the small arteries, or, it may be, affecting the capil- 

 laries, for it has been asserted that the epithelioid plates of the 

 capillaries vary in form according to the relative quantities of 

 carbonic acid and oxygen present in the blood. But this is not 

 the whole explanation of the matter, since similar variations in 

 resistance are met with when blood is driven through fine capil- 

 lary tubes of inert matter. In such experiments it is found that 

 the resistance to the flow increases with a diminution of the 

 oxygen carried by the red corpuscles, and is modified by the 

 addition to the blood of even small quantities of certain drugs. 



It is obvious, then, that in the living body the peripheral 

 resistance, being the outcome of complex conditions, may be 

 modified in many ways. Experiment teaches us that, even in 

 dealing with non-living inert matter, the flow of fluid through 

 capillary tubes may be modified on the one hand by changes in 

 the substance of which the tubes are composed, and on the other 

 hand by changes in the chemical nature (even independent of the 

 specific gravity) of the fluid which is used. In the living body 

 both the fluid and the tubes, both the blood and the walls of the 

 minute vessels, are subject to incessant change ; the vessels are 

 continually changing because they are living structures, and the 

 blood is continually changing not only because it too is in part at 

 least alive, but also because all the tissues of the body are working 

 upon it. The changes in the one, moreover, are capable of reacting 

 upon and inducing changes in the other ; and, lastly, the changes 

 both of the one and of the other may be primarily set going by 

 events taking place in some part of the body far away from the 

 region in which these changes are modifying the resistance to th* 

 flow. 



