134 THE VASCULAR SYSTEM 



is proved, says Thoma, by the microscopic observation of the axial 

 stream. The stream of corpuscles moves so fast that it appears 

 either completely homogeneous or faintly striated, both in the 

 artery and its branches (excluding the capillaries), and thus must 

 be flowing at the same rate, for the slightest diminution alters the 

 homogeneity. When the tongue of a frog is drawn forward from 

 the mouth of a curarised frog and pinned out for examination, 

 there follows at first considerable deviation from homonomous 

 ramification, owing to injury and local dilatation. Half-an-hour 

 later the tongue is hypersemic, but the flow is again of the same 

 rate in all the arterial ramification. 



The generalisation follows on these observations of Thoma 

 that the total cross sectional area of the arterial ramification 

 measured at any place is the same, 1 and that the sudden increase 

 occurs in the ramification of the capillaries, so that the rate of 

 flow is lessened therein from 500 mm. to i mm. or less a second. 

 This important generalisation, so opposed to accepted teaching, 

 requires confirmation or refutation. 



The capillary system in the tissues of man is like the blood 

 channel system of the Turbellarian worm, a vast system of tissue 

 spaces lined by cells which confine the corpuscles, and by shrinking 

 or swelling and by varying osmotic state regulate in part the 

 outflow and inflow of lymph. These cells are capable of phagocytic 

 action and of dividing and producing the primary type of leucocyte. 

 The arteries deliver the blood to these spaces, and many or few of 

 them are filled at any moment and in any tissue according as the 

 arteries dilate and increase the supply, or the venous flow is im- 

 peded by gravity or external pressure. It is absurd, therefore, to 

 calculate the size of the capillary bed from the relative velocity in 

 the aorta and in the capillaries of a transparent membrane. 



The cells both of the tissues and the capillaries swell or shrink 

 under -the influence of the solutions which bathe them, and thus 

 alter both the volume of the organ and the capacity of the vascular 

 system which is included in it. The tissue cells adapt themselves 

 slowly to altered osmotic conditions of the tissue fluids, and do 

 not by any means come into equilibrium with them. With the 

 return to normal physiological relations the recovery of the normal 

 concentrations of the cellular contents is rapid and complete. 

 The influence on the capillary circulation of osmotic and surface 

 1 Supposing the state of constriction to be the same. 



