AND BLOOD PRESSURE 133 



to be in the course of the developing blood channel as it pushed 

 its way among them, and were developed in response to the needs of 

 the vessel" 



The first capillaries in the area vascular of the developing 

 chick have their origin in the secretory activity of the cells which 

 first form vacuoles and finally networks of capillary spaces. It is 

 the functional activity of the cells which determines the rate of 

 flow and pressure within, and finally the structure of the vessels 

 and of the power of the heart. The increase in size of the lumen 

 of a vessel, says Thoma, depends upon the rate of blood flow. 

 Hence the dilatation of collateral pathways and establishment of 

 efficient anastomoses which follows the ligation of a main artery. 

 The growth in thickness of the vessel wall is dependent upon the 

 diameter of the lumen of the vessels and the blood pressure. The 

 tension in the circular direction of a tube is equal to the product of 

 the pressure by the radius. The longitudinal tension is equal to 

 half this product. The wider channels first formed in the area 

 vasculosa thus develop into arteries ; collateral capillaries develop 

 into arteries when a main artery is tied ; and the thyroid vein 

 develops the structure of an artery when the central end of the 

 carotid is joined end to end on to it. 



Thoma states that at all points where the transverse sectional 

 aieas of the lumina of the arterial trunks has been investigated 

 the sum of the sectional areas of the main trunks is equal to that 

 of their branches. Thus the sectional area of the ascending aorta 

 equals the sum of that of the two carotids, and subclavians and 

 the descending aorta, together with such smaller branches as were 

 given off above the place of measurement. Similarly the sectional 

 area of the abdominal aorta taken 2 cm. above its bifurcation 

 is equal to that of the two common iliacs and the smaller 

 branches. The same law holds good, Thoma says, in the case of 

 the arterioles of the tongue or web of the frog. As, he says, the 

 transverse section of the main artery is equal to the transverse 

 section of all its branches, the average rate of flow is the same 

 in all arteries, and the quantity flowing through the transverse 

 section of any artery in a given time is proportional to the section 

 of the artery. This law only holds good so long as disturbing 

 influences due to vaso-dilatation and functional a'ctivity are in 

 abeyance. 



That the ramification of the small arteries is homonomous 



