12 • THE CIRCULATORY SYSTEM 



n Chapter 10 we saw how the nervous system develops in relation to the 

 structures that it supplies. It would seem that the circulatory system must 

 develop in a similar fashion, since each organ requires blood vessels running 

 to and from it, just as a limb has sensory and motor nerves carrying impulses 

 to and from it. As a matter of fact, if we watch blood vessels growing in vitro 

 in tissue culture, we see the ends of vessels pushing their way through the 

 mass of cells, retracting, and starting out in other directions. The general 

 impression is that of random growth similar to the growth of nerve fibers 

 in an unoriented medium. 



In the embryo organs may "attract" blood vessels. This may be shown 

 by transplanting organs into various foreign regions of the embryo. The 

 transplanted organ always becomes supplied with blood vessels, and these 

 vessels may be very different from those which provide the normal supply 

 to the organ in the normal position. Thus, there is a great deal of flexibility 

 in the development of the circulatory system, and for this reason it is not 

 surprising to find many anomalies in the adult. Blood vessels may take an 

 abnormal path to an organ. Sometimes a vessel which normally degenerates 

 persists in the adult. 



