CIRCULATION IN SPECIAL PARTS 241 



"the vena cava and the torcular there lies 110 appreciable resistance; 

 between the aorta and the cerebral capillaries or veins there lies the 

 resistance of the arterioles. 



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

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

 Every variation in respiration and every muscular movement is fol- 

 lowed by passive changes in the circulation of the brain. Compression 

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

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

 pressing on the jugular vein are sufficient to affect the cerebral circu- 

 lation. 



FIG. 127. To SHOW EFFECT OF STIMULATING PERIPHERAL END OF THE VAGUS ON 

 THE CEREBRAL PRESSURE. 



A, Carotid pressure; B, right auricle; C, intracranial pressure; D, torcular pressure. 



Every stimulus that enters the organism and affects the general 

 vaso-motor 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 

 vaso-motor centre ; the splanchnic vessels constrict, the blood -pressure 

 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 vaso-motor centre a protective mechan- 

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

 supplied to the brain. At the moment that excitation from the 



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