352 OF THE CIRCULATION OF THE BLOOD. 



its vessels, or the parts around them, which, being natural, permits the most 

 easy transit of the blood, but, being disturbed, increases the hindrances to 

 its passage." A physical principle has been put forth by Professor Draper, 1 

 which seems quite adequate to explain these phenomena. It appears fully 

 capable of proof, that " if two liquids communicate with one another in a 

 capillary tube, or in a porous or parenchymatous structure, and have for 

 that tube or structure different chemical affinities, movement will ensue ; 

 that liquid which has the most energetic affinity will move with the greatest 

 velocity, and may even drive the other liquid before it." Now Arterial 

 blood, containing oxygen with which it is ready to part, and being prepared 

 to receive in exchange the carbonic acid which the tissues set free, must 

 obviously have a greater affinity for those tissues than Venous blood, in 

 which both these changes have already been effected. Consequently upon 

 mere physical principles, the arterial blood which enters the Systemic capil- 

 laries on one side, must drive before it, and expel on the other side of the 

 network, the blood which has become venous whilst traversing it ; but if the 

 blood which enters the capillaries have no such affinity, no such motor power 

 can be developed. On the other hand, in the Pulmonary capillaries the op- 

 posite affinities prevail. The venous blood and the air in the cells of the 

 lungs have a mutual attraction, which is satisfied by the exchange of oxygen 

 and carbonic acid that takes place through the walls of the capillaries ; and 

 when the blood has become arterialized, it no longer has any attraction for 

 the air. Upon the very same principle, therefore, the venous blood will 

 drive the arterial before it in the Pulmonary capillaries, whilst respiration 

 is properly going on ; but if the supply of oxygen be interrupted, so that 

 the blood is no longer aerated, no change in the affinities takes place whilst 

 it traverses the capillary network; the blood continuing venous, still retains 

 both its need of a change, and its attraction for the walls of the capillaries; 

 and its egress into the pulmonary veins is thus resisted, rather than aided, 

 by the force generated in the lungs. The change in the condition of the 

 blood, in regard to the relative proportions of its oxygen and carbonic acid, 

 is the only one to which the Pulmonary circulation is subservient ; but in the 

 Systemic circulation, the changes are of a much more cemplex nature, every 

 distinct organ attracting to itself the peculiar substances which it requires 

 as the materials of its own nutrition, and the nature of the affinities thus 

 generated being consequently different in each case. But the same law may 

 be considered to hold good in all instances. Thus the blood conveyed to the 

 Liver by the portal vein, contains the materials at the expense of which the 

 -bile-secreting cells are developed ; consequently the tissue of the Liver, which 

 is principally made up of these cells, possesses a certain degree of affinity or 

 attraction for blood containing these materials ; and this is diminished, so 

 soon as they have been drawn from it into the cells around. Consequently 

 the blood of the portal vein will drive before it, into the hepatic vein, the 

 blood which has traversed the capillaries of the portal system, and which, in 

 doing so, has given up the elements of bile to the solid tissues of the liver. 2 

 272. The influence which the Nervous System is known to exert upon the 

 functions of Nutrition and Secretion, which are very intimately ivhih'd to 

 the movement of the blood in the Capillaries, would lead us to expect that 

 it should exorcise some like influence over that movement itself. And two 

 distinct channels for such an influence maybe assigned with much probabil- 

 ity ; first, the control exercised by the Sympathetic system over the diameter 



Treatise on the Forces which produce the Organization of Plants, pp. 'Ji>-41. 

 ' Vnr further information, the render is referred to Mr. Savory's Review of the 

 whole of this Mihjeet. with original experiments, in the Brit, and For. Med.-Chir. 

 Ruvicw, vol. xv, p. 372, and vol. xvi, p. 12. 



