MOVEMENT OF THE BLOOD IN THE ARTERIES. 



335 



a gradual diminution of pressure, indicated by the diagonal line R c, may be 

 observed as the outlet is approached. Now, if the bore of the tube be con- 

 tracted at any part, as at o, an obstacle to the discharge of the fluid will be 

 created, through the increase of friction; the pressure against the walls of 

 the preceding portion of 

 the tube R to o will increase, 

 and consequently the line of 

 levels will rise in that part, 

 as indicated by the dotted 

 line R b c. If, on the other 

 ha nd, the outlet be enlarged, 

 the fluid will be discharged 

 more easily, and the line of 

 levels will fall with great 

 rapidity. Upon these prin- 

 ciples M. Marey 1 has con- 

 structed an apparatus bear- 

 ing some resemblance to the 

 different systems of vessels 

 in the body, a constricted 

 portion at o (Fig. 137) rep- 

 resenting the capillaries, 2 

 and being preceded by a 

 wider portion for the Arte- 

 ries, and followed by a still 

 wider one for the A r eins. By 

 this instrument it may be 

 shown physically, that the 

 pressure is highest in the 

 Arterial vessels, whilst it is 

 much less in theCapillaries, 

 and is lowest in the Veins. 3 

 It may also be shown that 

 the mean tension of an Ar- 

 tery diminishes in propor- 

 tion to its distance from the 

 heart, and is by so much 

 the less in proportion as the 

 capillaries are dilated, and 

 thus constitute .a smaller 

 obstacle to the passage of 

 the arterial blood. These 

 facts, at least as regards the 

 difference of the pressure in the arterial and venous system, were clearly 

 perceived by Hales, and having been substantiated by Bernard with the dif- 

 ferential Manometer, Fig. 138 ; the two mouthpieces of which, e e, being 

 inserted into Arteries at the same distance from the heart, as the two caro- 



1 Brown-Sequard's Jmirn. de la Physiol., vol. ii, p. 436. See also Rutherford's 

 New Scheme of the Circulation, Journ of Anat. and Phys., 1872, vol. vi, p. 249. 



2 The capillaries of the body, though in their,agu;regate sectional area much larger 

 than the aorta, yet by reason of their smallness, and the consequent great increase of 

 friction against their walls, operate virtually as such a constricted portion. 



3 In the living body the pressure rises again in the veins, as their collective calibre 

 is much less than that of the capillaries, though greater than that of the arteries. 



4 Bernard, vol. i, p. 205, 1859. 



Diffjreutial Manometer of Bernard. 4 



