564 OF THE CIRCULATION OF BLOOD. 



results from their continued operation. Here, again, is another evidence, that 

 something different from mechanical power must be the agent, that operates 

 in all the foregoing cases. 



740. It appears, from the preceding facts, that the conditions, under which 

 the power in question uniformly operates, may be thus simply and definitely 

 expressed: Whilst the injection of blood into the Capillary vessels of every 

 part of the system, is due to the action of the Heart, its rate of passage 

 through those vessels is greatly modified by the degree of activity in the pro- 

 cesses, to which it should normally be subservient in them ; the current 

 being rendered more rapid by an increase in their activity, and being stagnated 

 by their depression or total cessation. Thus it seems that " the capillaries 

 possess a distributive power over the blood, regulating the local circulation, 

 independently of the central organ, in obedience to the necessities of each 

 part." If this be true, it is evident that the dilatation or contraction of the 

 Capillaries will only have a secondary influence on the movement of the blood 

 through them. The former condition is usually an indication of diminished 

 vital energy ; and when it is observed, it is almost invariably accompanied by 

 a retardation or partial stagnation of the current; on the other hand, the ap- 

 plication of a moderate stimulus, which excites the contractility, accelerates 

 for a time the motion of the blood, by rendering more energetic that reaction 

 between the fluids and the surrounding tissues, which is the condition that 

 really has the most influence over the current. That alterations in the chemi- 

 cal state of the blood (involving, of course, important changes in its vital pro- 

 perties) are capable of exercising a most important effect on the Capillary cir- 

 culation, is shown, not merely by the stagnation of the Pulmonary Circulation 

 in Asphyxia ( 780), but by the curious fact ascertained by Dr. J. Reid, that 

 the blood, when imperfectly arterialized, is retarded in the systemic capilla- 

 ries, causing an increased pressure on the walls of the arteries. He found that, 

 when the ingress of air through the trachea of a Dog was prevented, and the 

 Asphyxia was proceeding to the stage of insensibility, the attemps at inspi- 

 ration being few and laboured, and the blood in an exposed artery being quite 

 venous in its character, the pressure upon the Arterial walls, as indicated by 

 the haemadynamometer applied to the Femoral artery, was much greater than 

 usual. Upon applying a similar test to a Vein, however, it was found that 

 the pressure was proportionally diminished; whence it became apparent, that 

 there was an unusual obstruction to the passage of venous blood through the 

 systemic capillaries. After this period, however, the mercury in the ha?ma- 

 dynamometer applied to the artery began to fall steadily, and at last rapidly, 

 in consequence of the diminished force of the heart, and the retardation of the 

 blood in the pulmonic capillaries; but, if atmospheric air was admitted, the 

 mercury rose very speedily, showing that the renewal of the proper chemical 

 state of the blood, restored the condition necessary for its circulation through 

 the Capillaries. 



741. The principles already noticed ( 713) as put forth by Prof. Draper, 

 seem fully adequate to explain these phenomena. 



a. The arterial blood, containing oxygon with which it is ready to part, and being pre- 

 pared 1d receive in exchange the carbonic acid which the tissues set free. must obviously 

 have a greater alliniiy li>r the tissues, than venous Mood; in which both these changes have 

 already been eliccled. Consequently upon mere physical principles, the arterial Mood which 

 enters the .y.->temic Capillaries on one side, must drive before it, and expel on the other side 

 of the net-work, 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 Capillaries of the lungs the opposite allinities prevail. The 

 venous Mood and the air in the pulmonary cells have a mutual attraction, which is satisfied 

 by the exchange of oxygen and carbonic acid that takes place through the walls of the capil- 

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



