MOTION OF THE BLOOD IN THE CAPILLARIES. 565 



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 net-work; the blood continuing venous, still retains 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. 



c. 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 complex 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 differ- 

 ent in each case. But the same law holds 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-secret- 

 ing 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 has given up, 

 in doing so, the elements of bile to the solid tissues of the liver. The same principle holds 

 good in every other case. 



742. It can be scarcely doubted, that it is by some influence exercised over 

 the molecular actions, to which the Blood is subject in the Capillaries, that 

 the Nervous system can operate on the functions of Nutrition, Secretion, &c., 

 in the manner already alluded to (Chap, vn.) ; and this influence may be not 

 improperly termed vital, if by so designating it we merely imply that its 

 nature and mode of operation are unknown, but that it is closely connected 

 with those actions which are altogether peculiar to living beings. The follow- 

 ing experiment, made by Dr. Wilson Philip, exhibits in a convincing manner 

 the possibility of such an influence. " The web of one of the hind legs of a 

 frog was brought before the microscope; and while Dr. Hastings observed the 

 circulation, which was vigorous, the brain was crushed by the blow of a ham- 

 mer. The vessels of the web instantly lost their power, the circulation ceas- 

 ing ; an effect which cannot arise, as we have seen, from the ceasing of the 

 action of the heart. [Dr. P. here refers to experiments, by which it was ascer- 

 tained, that the circulation in the capillary vessels of the frog will continue for 

 several minutes, after the interruption of the heart's action.] In a short time 

 the blood again began to move, but with less force. This experiment was 

 repeated, with the same result. If the brain is not completely crushed, although 

 the animal is killed, the blow, instead of destroying the circulation, increases 

 its rapidity."* We are not hence to conclude, however, that the Nervous 

 system supplies any influence, which is essential to the. continuance of the 

 Circulation ; since it is only by such sudden and severe injuries to the nervous 

 centres, as instantaneously destroy the vitality of the whole system ( 735), 

 that the movement of the blood is arrested. The experiments of Muller and 

 others satisfactorily prove, that mere action of the Nerves does not produce 

 any direct effect upon the Capillary circulation; and this corresponds with the 

 well-known fact, that the Nutritive processes may continue as usual, after this 

 action has been suspended. All the facts, which bear upon the question of 

 the connection between Nervous agency and the forces maintaining the 

 Capillary Circulation, have an equal relation to the functions of Nutrition and 

 Secretion in general; and as already shown, the Nervous System also influ- 

 ences these, by the control it exerts over the diameter of the blood-vessels 

 ( 730). 



* Experimental Inquiry into the Laws of the Vital Functions. 4th edition, p. 52. 

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