636 CIRCULATION OF BLOOD AND LYMPH. 



by a greater blood-flow through the brain, so long as the arterial 

 pressure remains above intracranial pressure. Whether the brain 

 increases in volume as a result of a rise of arterial pressure is, on the 

 physiological side, unimportant; the main point is that the amount 

 of blood flowing through it is increased under such circumstances as 

 would cause a like result in other organs. That the compression of 

 the veins does not produce any sensible obstruction to the blood- 

 flow may be understood easily. In the first place, this compression 

 does not take place at the narrow exit from the skull, since at that 

 point the sinuses are protected from the action of intracranial press- 

 ure. The compression takes place doubtless upon the cerebral veins 

 emptying into the sinuses, and at this point the venous bed, 

 taken as a whole, is so large that the expansion due to an 

 ordinary rise of arterial pressure is distributed and has but little 

 effect on the volume of the flow. Secondly, very great increases in 

 arterial pressure, up to the point of rupture of the walls, have less 

 and less effect in actually expanding the arteries ; a point is reached 

 eventually at which these tubes become practically rigid, so that 

 farther expansion is impossible. This, of course, is true for every' 

 organ. 



The Regulation of the Brain Circulation. It is still a matter 

 of uncertainty whether the arteries of the brain possess vasomotor 

 nerves. Most of the authors who have studied the matter experi- 

 mentally have concluded that there are none.* These authors were 

 unable to show that stimulation of any of the nerve paths that 

 might innervate the brain vessels causes local effects upon the brain 

 circulation. Whenever such stimulations caused a change in pres- 

 sure or amount of flow in the brain the result was referable to an alter- 

 ation of general arterial pressure produced by a vasomotor change 

 elsewhere in the body. When as a result of such stimulation the 

 pressure rises in the circle of Willis, one may infer that if this is due 

 to a local constriction in the cerebral arterioles there should be a 

 fall of pressure in the venous sinuses and a diminished flow of blood; 

 if, on the contrary, it is due to a constriction elsewhere in the body 

 that has increased general arterial pressure, but has not constricted 

 the brain circuit, then there should be a rise in venous pressure and 

 intracranial pressure, together with a greater flow of blood through 

 the brain. Most observers obtain this latter result. Some inves- 

 tigators, Hiirthle, Francois-Franck, and others, f on the other 

 hand, have obtained results, especially from stimulation of the 

 cervical sympathetic, which indicated local vasoconstriction or vaso- 

 dilatation in the brain. So far as the experimental results for or 



* See Roy and Sherrington, Bayliss and Hill, Hill, Gaertner and Wagner, 

 loc. cit., and Hill and MacLeod, "Journal of Physiology," 26, 394, 1901. 



t Hiirthle, "Archiv f. die gesammte Physiologic," 44, 574, 1889; Fran^ois- 

 Franck, "Archives de physiol. normale et pathologique " 1890; Weber 

 "Archiv f. Physiologic," 1908, 457. 



