EL OD-PEESS URE. 



319 



Internal Friction. If tlie blood is studied as it is flowing 

 through a small artery in the Aveb of a frog's foot, it will be seen 

 that in the center of the current it is flowing much faster than at 

 the sides; this is the axial stream, and in it will be observed the' 

 red corpuscles. That portion of the current which is between the 

 axial stream and the walls of the vessel moves more slowly, 

 the rate diminishing from the center outward, until at the walls 

 themselves it is at the minimum. This outer portion is known as 

 the inert layer. It should be stated that this arrangement of the 

 current is not due to any peculiarity of the blood or of the vessels 

 through which it flows, but is present in every fluid while flowing 

 through a tube. Between the different layers of fluid there is 

 friction, called internal friction. The smaller the tubes the greater 

 the internal friction, so that the amount of friction in the sub- 

 divisions of the aorta and its numerous ramifications is very 

 great, and this friction acts as an obstacle to the outflow of. the 

 blood, constituting peripheral resistance. 



BLOOD-PRESSURE. 



The systemic circulation of the blood i. e., its flow from the 

 left ventricle through the arteries and capillaries ; and back by the 

 veins to the right ventricle again is a movement from a point of 

 high pressure, the left ventricle, to one of low pressure, the right 



r.a. 



FIG. 165. Height of blood-pressure (b.p.) in left ventricle (l.v.) : a, arteries; c, 

 capillaries ; v, veins ; r.a., right auricle ; o o, line of no pressure (after Starling). 



auricle. This is shown in Fig. 165, where the pressure is greatest 

 at the left ventricle, gradually diminishing in the large arteries, 

 until at the end of the arterial system the fall is abrupt ; it falls 

 gradually throughout the capillaries and veins until the large veins 

 in proximity to the heart are reached, where it is negative (p. 323). 

 The fact that the blood within the vessels is under varying 

 degrees of pressure may be demonstrated by repeating the classic 

 experiment performed by Stephen Hales, an English Episcopal 

 clergyman, and described by him in " Statical Essays, Containing 

 Hsemostaticks." This was "published in London in 1733. He in- 



