3 o8 TEXT-BOOK OF PHYSIOLOGY. 



the arteriole region. The fall in pressure, however, is not great in 

 the larger vessels of the arterial system. It is only in the smallest 

 arteries, before their passage into the capillaries, that an abrupt fall 

 in pressure takes place. 



The Capillary Pressure. The small size of the capillaries pre- 

 cludes an investigation of their pressure by manometric methods. 

 It may be stated, however, to be approximately equal to the pressure 

 required to obliterate their lumen and to whiten the skin. The 

 apparatus of v. Kries is based on this theory. A small glass plate, 

 from 2.5 to 5 sq. mm., is fastened to the under surface of a support 

 of suitable size carrying a small scale pan. The glass plate is placed 

 on the skin near the root of a finger-nail and the scale pan gradually 

 weighted until the vessels are obliterated, as shown by the blanching 

 of the skin. From results obtained with this apparatus v. Kries 

 estimated the pressure in the capillaries of the hand at 37 mm. Hg, 

 and in the ear at 20 mm. 



The Venous Pressure. In passing from the capillaries to the 

 heart the pressure continues to fall. The increasing size of the 

 veins permits again of manometric observations in different regions. 

 In the crural vein the pressure has been found to be equal to 14 mm. 

 Hg, and in the brachial vein 9 mm. In the jugular and subclavian 

 and other vessels near the heart it is zero or even negative; that is, less 

 than atmospheric pressure to the extent of from i to 10 mm. mercury. 



The amount and relation of the pressures in the three divisions of 

 the systemic vascular apparatus are approximately shown in Fig. 

 144. 



Causes of the Blood-pressure. In correspondence with the 

 laws of the flow of fluids through both rigid and elastic tubes, the 

 flow of blood through the blood-vessels under the driving-power of 

 the heart encounters friction. ^This is to be sought for not between 

 the blood and the inner surface of the vessel, but rather in the cohesion 

 of the particles of blood./ This it is which offers resistance to the 

 onward movement of the blood and which must be overcome if the 

 circulation is to be maintained. Close to the inner surface of the 

 vessel there is a layer of blood which is motionless, known as the 

 still layer, caused by an adhesion between the blood and the vessel. 

 Between this layer and the axis of the stream there is an infinite num- 

 ber of layers. The cohesion between these layers gradually dimin- 

 ishes in passing from the periphery to the center of the stream. The 

 larger the stream, the less is the cohesion, and the reverse. In the 

 large arteries the still layer is small in amount as compared with the 

 total volume of blood passing through them; hence the axial cohesion 

 is readily overcome and the friction is but slight. In the smallest 

 arteries and capillaries the ratio changes and the friction rapidly 

 increases and to a considerable extent. In the veins the ratio again 



