ARTERIAL CIRCULATION. 1 13 



walls. In vessels of a certain calibre this movement is jerk- 

 ing and rhythmical, precisely like that of the heart, and if we 

 place the finger on the course of an artery we feel the shock 

 of the blood, or the pulse. The pulse and the beating of 

 the heart are synchronous, that is, they take place simultane- 

 ously, or rather with an interval so short as to be impercep- 

 tible. But in proportion as the blood advances through the 

 arterial ramifications, the numerous changes of direction which 

 it undergoes and the friction between it and the walls of the 

 vessels diminish the force of its impulsion; and at last, on 

 reaching the capillary vessels it flows continuously and with- 

 out shock. 



In examining, under a microscope, a vascular membrane 

 belonging to a living animal, the circulation of the blood 

 through the capillaries is plainly visible. The largest of 

 these canals allow the column of blood to pass rapidly, in 

 the smallest ones its course is slow and the blood-globules 

 can only pass one by one; they float in a transparent fluid, 

 and sometimes a globule becomes entangled obliquely across 

 the calibre of the vessel and stops until another comes to 

 push it forward. Malpighi was the first to verify in this 

 manner the accuracy of Harvey's theory, forty years after it 

 had been propounded by the illustrious English physiologist. 



The different causes, therefore, which accelerate or retard 

 the contractions of the heart, influence the motion of the 

 blood in the arteries; and farther, the contractility of these 

 vessels may be influenced by local causes, and the movement 

 of the blood is modified so as to be either retarded or 

 quickened, as they are contracted or relaxed. In the first 

 case the afflux of blood is not sufficient to excite the organs, 

 which become sluggish and partially paralyzed; in the second 

 it is so great as to cause an abnormal activity of the function. 

 And lastly, we all know that the repose or action of the 

 muscles has the effect of retarding or accelerating the circula- 

 tion, general or local, which, in the course of time, results in 

 the diminution or increase of the muscular strength. 



It is in the capillaries, in fact, that arterial blood yields to 

 the tissues the elements of which it is composed, and which 

 it delivers to them for assimilation, in order to receive in 



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