DISEASES OF ARTERIES AND VEINS. 245 



progressive growth of the organ transfers its boundary-line 

 from a — /? to y — 8, the capillary loop a — h will no longer suffice to 

 carry on the nutrition of the extended vascular territory. A new 

 capillary loop, c — d^ is produced. Like every vessel in the organism, 

 this springs from a pre-existing vessel. Suppose, what is quite 

 possible, that its two extremities are implanted into the capillary 

 loop a — Z>, in just the same way as the loop a — h is connected 

 with its parent-vessel. The enlargement of the blood-path 

 entails an increase in the amount of blood, so that the current 

 passes through c — (Sunder the same pressure, velocity and tension 

 as it did through a — h. But these values do not continue the 

 same for a — h. Taking a transverse section through a, we see 

 at once that through it flows not only the blood which fills the 

 capillary loop a — h under a pressure, velocity and tension each 

 of which equals unity, but also the blood which goes to fill the 

 loop c — d. Assuming that c — d equals a — h both in length and 

 sectional area, the pressure, velocity and tension at a must each 

 equal two. Continuing this train of reasoning, and assuming 

 that with the farther growth of the organ, fresh capillary vessels 

 are again and again projected from the old ones, the pressure, 

 velocity and tension must increase proportionately in the afferent 

 and efferent vessels ; greater demands will be made on the 

 resisting power of the transverse sections a and h. We might 

 accordingly expect to find the vessel increasing in calibre and its 

 walls growing progressively thinner. It does in fact increase in 

 calibre, but its wall does not grow thinner; a peculiar law, 

 which regulates the growth of the entire vascular system, comes 

 into operation at this point, a law according to which any in- 

 creased demand upon the resisting power of the transverse 

 section of a vessel, is met by a proportionate thickening of its 

 walls (hypertrophy and dilatation). The wall of the vessel 

 grows thicker, one limb of the capillary loop becoming an 

 afferent artery, the other an efferent vein.* 



* Strictly speaking, our scheme is only applicable to cases in which 

 a capillary area is provided with a single afiferent and efferent vessel, 

 i.e. to reiia mvrahilia. In order to make it suit all the varieties of 

 circulatory distribution throughout the body, it is necessary to add that 

 anastomoses between neighbouring capillary areas render the forma- 

 tion of larger and finally of absolutely large vascular territories possible, 

 the artery of one area bringing enough blood for both, while one vein 



