THE CAPILLARY CIRCULATION 221 



a rapid flow of blood through dilated arteries, and almost rigid 

 vessels, arteries, capillaries, and veins, all at the full or nearly full 

 arterial pressure. Similarly, in an inflamed area, by the inhibition 

 of fluid in the damaged tissue cells which are confined by connective 

 tissue, the veins are compressed and narrowed, and the arteries being 

 dilated, the capillary pressure rises, and the whole part throbs with 

 the pulse and receives a rapid flow of blood. If the swelling is too 

 great, strangulation of the circulation occurs, and the surgeon's knife 

 is required to relieve tension and promote flow. 



Rate of Flow. The velocity of a blood-corpuscle in the capillaries 

 of a frog's muscle has been reckoned to be 0-28 mm. per second. The 

 method most conveniently used is to empk>3 r an ocular micrometer, and 

 follow the course of a corpuscle during a period of time given by 

 a clock beating one-fifth seconds. The velocity has thus been found by 

 various observers to be 0-25 to 0-57 millimetre per second in cold- 

 blooded animals. 



By the entoptic method the velocity in the retinal capillaries has 

 been calculated to be 0-75 millimetre per second. With suitable 

 illumination of the eye the corpuscles are seen by the subject on a 

 ground-glass screen held 11 to 16 centimetres from the eye. A 

 corpuscle can be followed 20 to 30 millimetres on the screen. 

 Knowing the distance of the screen from the anterior nodal point 

 of the eye (A), the distance of the retina from the posterior nodal 

 point (B), and the distance travelled by the corpuscle on the 

 screen (C), the real distance x travelled in a given time can be cal- 

 culated. 



As the red corpuscles travel in the axial part of the stream, and as 

 the mean velocity in any tube equals one-half the axial velocity, the 

 true mean velocity of flow is less than the above. It can be taken to 

 be about 15 to 30 millimetres per minute, and in the smallest capil- 

 laries, where the flow is often obstructed, it is still less. Since the 

 velocity stands in inverse proportion to the sectional area at any 

 point in a system of tubes, the proportional relationship of the total 

 sectional area of the capillaries to that of the aorta can be reckoned 

 if we know the mean velocity in the capillaries and in the aorta. Thus, 

 if the mean velocity be taken as 500 millimetres per second in the 

 aorta, and 0-5 millimetre psr second in the capillaries, the relation 

 is 1 =1,000. In man the sectional area of the aorta is 4-4 square centi- 

 metres. The total sectional area of the capillaries filled with blood 

 at the lim is thus equal to 4,400 square centimetres. This result is, 

 of course, only approximate. 



The Capillary Blood-Pressure. -The measurement .of the capillary 

 pressure has been attempted by placing a glass plate 2-5 to 5 square 

 millimetres in size on the skin in a suitable place, such as on the last 

 joint of the finger. From this glass plate hangs a small scale-pan. 



