336 CIRCULATION OF NUTRITIVE FLUID. 



when the heart's action is enfeebled or partially interrupted ; and it 

 would thus appear, that the local influences by which they are produced, 

 are overcome by the propelling power of the central organ, when this 

 is acting with its full vigour. When the whole current has nearly stag- 

 nated, and a fresh impulse from the heart renews it, the movement is 

 seldom uniform through the entire plexus supplied by one trunk ; but is 

 much greater in some of the tubes than in others, the variation being 

 in no degree connected with their size, and being very different in its 

 amount at short intervals. 



597. All these circumstances indicate that the movement of blood 

 through the Capillaries is very much influenced by local forces ; although 

 these forces are not sufficiently powerful, in the higher animals, to 

 maintain it alone. And from other facts it appears, that the condi- 

 tions necessary for the energetic flow of blood through these vessels, 

 are nothing else than the active performance of the nutritive and other 

 operations, to which they are subservient. The examination of a single 

 one of these processes, will afford us the requisite proof. The blood 

 when circulating through the systemic capillaries, yields a portion of its 

 oxygen to the tissues it permeates, and receives from them carbonic 

 acid. On the other hand, when passing through the pulmonary capil- 

 laries, it gives up its carbonic acid to the atmosphere, and imbibes a 

 fresh supply of oxygen. Now if either of these changes be prevented 

 from taking place, a retardation and even a complete stagnation of the 

 blood will take place, the flow through the capillaries being now 

 resisted, instead of accelerated, by the relation which the blood bears to 

 the tissues. Thus it has been shown, that if an animal be partially 

 deprived of oxygen, so that the arterial blood is not duly aerated (rather 

 resembling the ordinary venous blood), and cannot exert its proper 

 action on the tissues, the pressure upon the walls of the systemic 

 arteries is increased, although the supply of blood propelled by the 

 heart, and the propulsive power of the heart itself are diminished; and 

 this plainly indicates a retardation in the systemic capillaries, producing 

 an undue accumulation in the arteries. On the other hand, the sus- 

 pension of the supply of oxygen to the lungs, either by an obstruction 

 in the air passages, or by the substitution of some other gas, brings the 

 pulmonary circulation to a stand in a very short time, the blood not 

 being able to undergo its usual changes in the capillaries of those organs; 

 and by this stagnation, the whole movement of blood is speedily checked. 

 The readmission of oxygen, if the suspension of the circulation have 

 not been too long continued, occasions the renewal of the movement in 

 the capillaries, and thence in the whole circle of vessels ; and this even 

 after the heart has ceased to propel blood towards the lungs. 



598. The principles already noticed ( 547), as put forth by Prof. 

 Draper, seem fully adequate to explain these phenomena. The 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 the tissues, than venous 

 blood, in which both these changes have been already effected. Conse- 

 quently, upon mere physical principles, the arterial blood, which enters 

 the systemic capillaries on one side, must drive before it, and expel on 



