I.— PH YSIOLOGY , 159 



with changes in the activity of the tissues would be valueless without an 

 equally delicate co-ordination of the circulation, since on this depends 

 the transport of gases between the tissues and the lungs; nor would this 

 suffice unless the blood were of such a nature as to afford a suitable medium 

 for the carriage of oxygen and C0 2 . 



So far as our knowledge of the properties of the blood itself is concerned 

 we can point to a great deal of progress. Naturally enough, the blood of 

 many different animals has been investigated, but it has been found that 

 the dissociation or absorption curves, which express the relationship 

 between the amount of gas held in dissociable combination in the blood 

 and the concentration of that gas to which the blood is exposed, differ in 

 different species, and indeed in different individuals of the same species. 

 and this fact makes it imperative that in physiological work the properties 

 of the blood of the individual under consideration at the time should be 

 investigated. Correct inferences as regards the properties of human blood 

 from which we may deduce the exact part played by the blood in gas 

 transport cannot therefore be drawn from studies of the blood of lower 

 animals, and in consequence we find that more and more attention is being 

 directed to the experimental investigation of the blood of man under 

 widely varying conditions, both in health and in disease. 



I suppose that as much attention has been directed by physiologists to 

 the circulatory system as to any other branch of physiology, and yet when 

 we begin to question ourselves about the true functional regulation of the 

 circulation our information still seems to be amazingly hazy and indefinite. 

 Direct observations on the anaesthetized animal with the assistance of 

 recording instruments have established a number of perfectly definite, 

 facts. We have some notion of the mechanics of the circulatory system ; 

 we have ascertained the general course and distribution of the vaso-motor 

 nerves ; we have found that not only the arteries but even the capillaries 

 are capable of active contraction and dilatation ; we have identified the 

 powerful effect that may be exerted on the larger blood vessels and 

 capillaries by the products of the ductless glands and by substance? 

 produced in the metabolism of the tissues. We have gone farther than this : 

 we have by means of artificial methods of stimulation identified various 

 vascular reflexes ; we have found that the heart when isolated from the 

 body can deal with an increase in the rate at which venous blood is supplied 

 to it by altering its amplitude of beat without any variation in rate, though 

 if its nervous connections are intact an acceleration of the venous return 

 causes a reflex increase in the pulse rate, the antithesis of the reflex 

 retardation caused by an undue rise of arterial blood pressure. 



But what does all this seemingly exact knowledge amount to ? Are 

 we not really only showing up the potentialities in the circulatory system I 

 It may be argued that we shall gradually build up the whole as our know- 

 ledge of the component parts becomes more complete, but I retort — can 

 you build up the whole without a preliminary notion of its characteristic 

 qualities ? Surely before we attempt an explanation we must know what 

 it is that we are trying to explain. The circulation has to supply oxygen 

 and foodstuffs to the tissues, it has to remove CO a and other waste products 

 from them. Well, what are the claims made on the circulation in every lay 

 life ? We can answer that question in the case of man from our knowledge 



