236 THE VASCULAR MECHANISM. 



normal circulation can take place. When the tissue becomes affected, the 

 disturbance of the relations between the tissue and the blood may, as in the 

 later stages of inflammation, so augment the resistance that the passage of 

 the blood becomes at first difficult and ultimately impossible. And it is 

 quite open to us to suppose that under certain circumstances the reverse of 

 the above may occur in this or that area, conditions in which the resist- 

 ance may be lowered below the normal and the circulation in the area 

 quickened. Thus the vital condition of the tissue becomes a factor in the 

 maintenance of the circulation ; and it is possible, though not yet proved, 

 that these vital conditions are directly under the dominion of the nervous 

 system. 



171. Changes in the peripheral resistance may also be brought about by 

 changes in the character of the blood, especially by changes in the relative 

 amount of gases present. When a 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 in- 

 stance by the amount of outflow in relation to the pressure exerted, varies 

 considerably owing to changes taking place in the organ, and may be in- 

 creased 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, atropine, etc., 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 capillaries, 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 capillary 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. Ex- 

 perience 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. 



CHANGES IN THE QUANTITY OF BLOOD. 



172. In an artificial scheme changes in the total quantity of fluid in 

 circulation will have an immediate and direct effect on the arterial pressure, 

 increase of the quantity heightening and decrease diminishing it. This 

 effect will be produced partly by the pump being more or less filled at each 

 stroke, and partly by the peripheral resistance being increased or diminished 

 by the greater or less fulness of the small peripheral channels. The venous 



