340 INFLUENCE OF QUALITY OF THE BLOOD. [BOOK i. 



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, the blood, and the walls of the minute vessels, 

 being both alive, are incessantly subject to change ; 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 the flow. 



