THE CEREBRAL CIRCULATION. 



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abdominal veins be compressed, venous blood can be driven out of the 

 torcular in a continuous stream. If in the living animal the torcular 

 venous pressure and the vena cava pressure be simultaneously recorded 

 in manometers placed side by side, and if at the same time the abdomen 

 be compressed, then the pressure rises in both by equal increments, 

 when the arterial pressure remains constant, and by greater increments 

 in the torcular if the arterial pressure rises also. Between the vena 

 cava and the torcular there lies no appreciable resistance, between the 

 aorta and the cerebral capillaries or veins there lies the unknown 

 resistance of the arterioles. Thus, the variations of arterial pressure, 

 read in millimetres of mercury, only produce variations in cerebral 

 venous pressure which can be read in millimetres of water. 



Every change in the position of an animal, owing to the influence of 

 gravity on the vascular system, affects the cerebral circulation. Every 



1 



3 



Fig. 89. — Effect of asphyxia. Curarised dog. A, aortic pressure ; B, cerebral venous 

 pressure ; C, vena cava pressure. 1, fall of C and B due to cessation of artificial 

 respiration ; 2, asphyxial rise of A and B ; 3, rise of C and B and fall of A ; 4, re- 

 establishment of artificial respiration. Note. — The tambour used for recording B was 

 more delicate than that used for recording C. — Bayliss and Hill. 



variation in respiration and every muscular movement is followed by 

 passive changes in the circulation within the brain. Compression of the 

 jugular veins or of the abdomen causes a marked rise in cerebral venous 

 pressure. The movements of the muscles of the neck by pressing on the 

 jugular vein are sufficient to affect the cerebral circulation. 



Every stimulus that enters the organism and affects the general 

 vasomotor centre produces a passive effect on the cerebral circulation. 

 It is by means of the great splanchnic area that the blood supply to 

 the brain is controlled. An anaemia of the spinal bulb excites the 

 vasomotor centre, the splanchnic vessels contract, the blood pressure 



