328 TIIK HK(,l.\MN(,S OF CAl'SAI. MORPHOLOGY 



i.e., by increased diffusion, and their power to exhibit a 

 chemically conditioned cytotropism, which causes the sprouts 

 to find one another and unite. A similar process can be 

 directly observed in isolated segmentation-cells, which tend 

 to unite in consequence of a power of mutual attraction. 



"(4) The faculty of developing normal arterial walls in 

 response to strong intermittent pressure, and normal venous 

 walls in response to continuous lesser pressure." It has 

 been shown, for instance, by Fischer and Schmieden that 

 in dogs a section of vein transplanted into an artery takes 

 on an arterial structure, at least as regards the circular 

 musculature, which doubles in thickness. 



" (5) The power to regulate the normal ] length of the 

 arteries and veins, in adaptation to the growth of the 

 surrounding tissues, in such a way that the stretching action 

 of the blood-stream brings the vessel to its proper functional 

 length. 



"(6) The power to form, in response to slight increases in 

 longitudinal tension, new structural parts which take their 

 place alongside the existing longitudinal fibres. 



" (7) The power to regulate the width of the circular 

 musculature according to the degree of food-consumption by 

 the tissues, in response to nerve impulses initiated in these 

 tissues. 



"(8) The power possessed by the circular musculature of 

 responding to such continuous functional widening, by the 

 formation of new structural parts in the circular musculature, 

 and so of widening the vessel permanently or by this new 

 formation of muscular fibres thickening the circular muscu- 

 lature. 



"(9) The faculty of being stimulated by increased blood- 

 pressure to produce the same structural changes as men- 

 tioned in par. S, though here the response is otherwise 

 conditioned " (pp. 126-7, 1910). 



It is by virtue of the tissue-properties detailed above that 

 the complex functional adaptations of the blood-vessels 

 come about. 



The development of the vascular system is no mere 

 automatic and mechanical production of form, apart from 

 1 That is, the length they take up when .separated from the body. 



