148 



THE MECHANISM OF THE CIRCULATION. 



rises, and more blood is driven through the brain. At the same time, 

 the respiratory centre is excited, and by the increased action of the 

 respiratory pump more blood is driven to the right heart, and thence to 

 the brain. We have in the vasomotor centre a protective mechanism, 

 by which blood can be drawn at need from the abdomen and supplied to 

 the brain. At the moment that excitation from the outside world 

 demands cerebral response, the splanchnic area contracts, and more 

 blood is driven through the brain. The brain has no direct vasomotor 

 mechanism, but its blood supply can be controlled indirectly by the 

 vasomotor centre acting on the splanchnic area. The vasomotor 

 centre is part of the central nervous system, and feels the same needs 

 and is stimulated by the same centripetal impulses as affect the rest 

 of that system, and thus it maintains a supply of blood to the central 

 nervous system which corresponds to its functional activity. 



The arterial supply to the brain is in the lower animals so super- 

 abundant, that, in the dog, 

 four of the arteries — two 

 common carotids and two 

 vertebrals — which supply 

 the brain can be tied in the 

 course of ten minutes, and 

 yet the animal either at once, 

 or after a temporary period 

 of idiocy and paralysis, com- 

 pletely recovers. In the 

 monkey, on the other hand, 

 ligation in one operation of 

 one vertebral artery, in addi- 

 tion to the two carotids, pro- 

 duces either death in twenty- 

 four hours or softening of the 

 great brain, accompanied by 

 idiocy and paralysis of move- 

 ment and sensation. The 

 efficiency of the anastomosis 



A, aortic pressure ; B, cerebral venous pressure ; through the circle of Willis 



Fig. 90.- 



-Erl'ect of drawing a 



ligature gently round 



, the neck, after placing a cannula in the trachea 



a 1 



varies 



in different indi- 



C, vena cava pressure. — Bayliss and Hill. 



viduals. Sudden compression of one common carotid artery in some 

 men produces epileptic spasm, and ligation of this artery has been 

 followed in some by more or less temporary paralysis on the opposite 

 side of the body : in others the effect is nil. 



The Pulmonary Circulation. 



The first physiologist who succeeded in making observations on the 

 pressure in the pulmonary artery was Beutner. 1 The more important 

 recent investigations on this subject are those of Bradford and Dean, 2 

 and Fran(jois-Franck. 3 By Bradford and Dean, dogs were placed under 

 the influence of morphia and curari, and artificial respiration was estab- 

 lished. The artery to the lower lobe of the left lung was reached by 



1 Ztschr.f. rat. Med., 1852, Bd. ii. S. 97. 



- Proc. Roy. Soe. London, 1889, vol. xlv. p. 362; Journ. Physiol., Cambridge and 

 London, 1894, vol. xvi. p. 34. 



3 Arch, dephynol, norm, etpath,, Paris, 1896, pp. 178, 193, 



