ON THE MOTION OP THE BLOOD. 75 



that if, v. c. we place one leg over the other knee, we 

 find not only that it, but even a much greater weight, 

 may be raised by the pulsation of the popliteal. Hence 

 an alternate systole and diastole, corresponding with 

 those of the heart, have long been assigned to them. 

 But this, although commonly believed on the evidence 

 of sense, is open to much question : * it may be asked, 

 especially, whether this pulsation is referrible to the 

 power of the artery, or only to the impulse given by 

 the heart to the blood propelled into the aorta. 



130. And indeed, after all, it appears that the dias- 

 tole of an artery is owing to the blood, — to a lateral 

 distention given by the impetus of the blood, so that 

 the coats are expanded ; and the vessel, by its elasti- 

 city, the next moment reacquires its natural thickness. 

 To the same impulse may be ascribed the lateral mo- 

 tion of the axis, observable in the larger arteries, if 

 serpentine and lying in loose cellular substance. (F) 



The genuine systole, produced by a contraction of 

 their substance, scarcely occurs, probably, while the 

 heart acts with sufficient vigor; but when they are un- 

 usually stimulated, or if the action of the heart fails or 

 is impeded by severe disease, then indeed the arteries 

 may supply its place and propel the blood by their own 

 vital energy, 



131. Since Whyttf and other illustrious physiolo- 



* T. Kirkland, Inquiry into the present state of Medical Surgery. London. 

 1/83. 8vo. vol. i. p. 306' sq. 



•f* Physiological Essays, containing an inquiry into the causes which promote 

 the circulation of the fluids in the very small vessels of animals, Ike. &C. 

 Second Edition. Edinb. 1761. 12mo. 



H. v. d. Bosch, Tiber das Mvskelvermfigen der Hnargtfassgen. Munster. 

 1786. 8vo. 



