CHAPTER XXIX 



PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 



Up to the present we have been considering the circulation of the blood 

 from a general point of view. There are certain organs and tissues, how- 

 ever, in which the general mechanism is altered in order to meet pecu- 

 liar requirements of blood supply. Thus, it is evident that the brain, 

 incased as it is in the rigid cranium, will be unable to contract and 

 expand as a result of vasoconstriction or vasodilation. On the other 

 hand, we know that the blood supply to this organ does vary con- 

 siderably from time to time. What is the nature of the mechanism by 

 which such changes are brought about? In the case of the liver the cir- 

 culation is peculiar on account of the fact that blood is carried to the 

 organ by two vessels, in one of which it is supplied under high pressure 

 and in the other, under low pressure. We must investigate the rela- 

 tionship of these two sources of blood supply. The circulation through 

 the coronary and pulmonary vessels must likewise receive special atten- 

 tion on account of the highly specialized functions of these organs. 



THE CIRCULATION IN THE BRAIN 



Anatomical Peculiarities 



Serious curtailment of the blood supply to the brain is guarded against 

 by the existence of the circle of Willis. Besides the four main arteries 

 the vertebrals and the two carotids the spinal arteries contribute to 

 the blood supply of the circle, and consequently in certain animals, such 

 as the dog, the four main arteries may be ligated without causing death. 

 In man, however, ligation of both carotids is usually fatal. The free 

 anastomosis displayed in the circle of Willis is not maintained in the 

 case of the arteries which run from it to supply the brain structure. On 

 the contrary, these vessels are more or less terminal in character; that 

 is to say, the capillary systems of the different vessels do not freely anasto- 

 mose, so that obstruction of one vessel, or an important branch, is followed 

 by death of the supplied area. The vessels which go to the pia mater, how- 

 ever, break up into numerous smaller branches, which freely anastomose 

 before entering the brain tissue. 



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