92 THE CIRCULATION OF THE BLOOD 



But the rise of pressure which occurs on injection of 

 adrenahn after the vagi have been cut is not entirely due 

 to vaso-constriction. Adrenahn has also a direct action 

 upon the heart, quickening it and increasing the amphtude 

 of each beat. To the diminished output from the arteries 

 is therefore added increased output from the heart. 



THE CIRCULATION IN THE CAPILLARIES 



We have seen that the arterioles, owing to their muscular 

 walls, present to the flow of blood a resistance which can 

 be varied by nervous and chemical means. The terminal 

 arterioles lead into the capillaries — dehcate tubes, about 

 0-5 mm. in length, composed of a single layer of flattened 

 endothehal cells. These capillaries he in a bed of lymph 

 which separates them from the tissue-cells. 



In the mesentery of the frog the circulation in the 

 capillaries can be readily observed and compared with the 

 circulation in the arterioles. In the latter it will be seen 

 that the red corpuscles, owing to their greater specific 

 gravity, run in the axis of the vessel where the stream is 

 fastest. Surrounding the corpuscular column is a clear 

 layer composed of plasma. Here the white corpuscles can 

 be seen rolhng in a leisurely manner along the inner wall 

 of the tubes. When the capillaries are reached, owing to 

 the narrowness of these vessels there is only one layer, 

 the corpuscles passing one by one. Here the blood flows 

 with great irregularity, stopping and rushing on alter- 

 nately. There is no pulsation, this having been effectively 

 damped by the terminal arterioles. Here, where the blood- 

 flow is at its slowest, occur the transference of food material 

 from the blood across the lymph to the cells, and of waste 

 products from the cells to the blood, the exudation of 

 lymph, and the migration of leucocytes into the tissue 

 spaces. At present, however, we are concerned not with 

 these nor Avith the dramatic changes which occur as the 

 result of injury, but only with such modifications in the 



