190 ESSENTIALS OF PHYSIOLOGY. 



fact that the specific gravity of the red cells is higher than that of the 

 plasma. The lumen of the capillaries is so small that no axial zone 

 is present. The velocity of the blood flow in the capillaries can be 

 directly observed with the aid of an eye-piece micrometer by noting 

 the time taken by a red corpuscle to travel a given distance, and varies 

 from 0'5 to O8 mm. per second; and as the average length of a 

 capillary is from 0'4 to 0'8 mm., any one corpuscle traverses the 

 capillary in one second. During this time, the interchange of oxygen 

 and of carbonic acid and of nutritive and waste material between the 

 blood and the tissues takes place through the capillary wall. The 

 capillary pressure is intermediate between that in the arteries and 

 veins, and since the capillaries open directly into the veins the 

 pressure in them is very easily influenced by a rise or fall in venous 

 pressure. 



If an irritant, e.g. dilute acetic acid, is applied for a moment or two 

 to the surface of th'e frog's mesentery, the minute arterioles and the 

 capillaries soon become dilated, and the blood flows more rapidly through 

 them. Presently the leucocytes begin to adhere to the capillary walls, 

 and some of them make their way through the interstices between the 

 epithelial cells ipto the surrounding tissue. At the . same time, the 

 capillary wall seems to alter so as to offer more resistance to the flow of 

 blood ; and the flow slackens and may cease, although the capillaries 

 and arterioles are still dilated. After a time, the capillaries become 

 filled with a mass of red and white cells, much of the plasma passing 

 through the walls of the capillaries into the lymph. Sometimes the 

 red corpuscles are also forced through the capillary walls, this being 

 called diapedesis. This series of events forms part of the process of 

 inflammation, which is defined as the response of the tissues to an 

 injury provided the latter does not cause death at once. The injury 

 may be mechanical or may be brought about by chemical substances, 

 including bacterial products. 



THE TIME OF THE CIRCULATION. ^ 



One of the best methods for determining the circulation time is to 

 inject into one jugular vein a strong solution of methylene blue ; the 

 jugular vein on the opposite side is exposed and allowed to rest on a 

 strip of white paper. The interval between the injection and the 

 moment at which the blue colour becomes visible in the opposite vein 

 is observed ; it varies from K^o20 seconds. 



Another method consists in sending a compensated electric current 

 through a section of an artery, e.g. the left carotid, and through a galvano- 

 meter. A little concentrated salt solution is injected into the opposite 



