CIRCULATION OF THE BLOOD. 129 



branes, or even masses of stucco, present no obstacle to the passage of 

 gases. The delicate wall of these cells, a tissue of almost inconceivable 

 tenuity, can offer no resistance. The gas passes in and out without im- 

 pediment or restraint. 



But though in this manner these little organisms perform their duty, 

 it is only for a time. They may take oxygen from the air- Summar of 

 cells and give it up in the system, and do this perhaps many the function of 

 thousand times, but it comes to an end at last. The inces- b 

 sant motion stops, and the worn and exhausted disc is brought to its term. 

 By degrees, as old age steals over it, it becomes corrugated and relaxed, 

 is unable to withstand chemical reagents, as its younger comrades can 

 do. Through the microscope it seems puckered and attenuated. The 

 red color of its interior deteriorates into a tawny tint. As with a leaf in 

 the autumn, the natural color of which disappears, and yellowness or 

 other change precedes its fall, so with the dying disc. Unable any 

 longer to discharge its duties, its existence is brought to a close, the de- 

 cayed ha^matin is shed out to give a transient tawny tint to the plasma, 

 but is presently strained off as one of the constituents of bile by the liver. 

 Nor is the illustration here used wholly metaphorical, for, in the case of 

 herbivorous animals, Berzelius has shown that the coloring matter of their 

 bile is identical with chlorophyll, the coloring matter of leaves. 



CHAPTER VIII. 



OF THE CIRCULATION OF THE BLOOD. 



The Heart as a Machine. Inadequacy of Harvey's doctrine of the Circulation. Physical Prin- 

 ciple of the Circulation ; applied in the case of a Nucleated Cell, Pervious Tissue, Motion of 

 Sap and of Blood. Dependence of the Circulation on Respiration. Forms of Circulation : 

 Systemic, Pulmonary, Portal. Description of the Heart : its Movements. Their Force, Num- 

 ber, and Value. Sounds of the Heart. Cause of its Contractions. Description of the Arte- 

 ries, Capillaries, Veins. Explanation of the Circulation of the Blood. Facts supporting it. 

 The First Breath. 



No function of the animal mechanism illustrates more strikingly the 

 doctrine that we must rely on physical agents for physiological explana- 

 tions than that which we have now to consider, the circulation of the 

 blood. 



We surrender some of the most beautiful recollections of classical 

 mythology, and some of the most cherished popular illusions The heart as 

 of our own times. The heart, which in the higher classes of an en s ine - 

 life is the central organ of impulse of the circulation, is to be degraded 

 into a mere engine. We have to speak of its valves, its cords, its pipes. 



I 



