146 THE MECHANISM OF THE CIRCULATION. 



circulatory system of the brain will assimilate itself more and more 

 to a scheme of rigid tubes. The velocity of the blood flow will be 

 increased, and the relative distribution of the blood in the arteries, 

 capillaries,. and veins will be changed. A rise of arterial pressure does 

 not by any means, as has been supposed, 1 produce an anaemia of the 

 brain, through compression of the cerebral capillaries and veins, but 

 rather it causes an increased velocity of blood flow. 



The intracranial pressure is, in all physiological conditions, the same 

 as the cerebral capillary and venous pressure. The intracranial tension 

 is, in fact, that tension which remains after the force of the heart has 

 been expended in driving the blood through the cerebral arterioles. 2 It 

 is therefore an ever-varying quantity. 



In normal conditions, with the animal in the horizontal position, the 

 intracranial pressure in the dog is generally equal to about 100 mm. H 2 0. 



There is no compensatory mechanism by which the brain matter 

 can be protected from great changes of circulatory pressure. In the 

 spasms of strychnine poisoning, the intracranial tension may rise to 

 40 or 50 mm. Hg. This is due, not only to the rise of arterial tension, 

 but to the enormous diminution of capacity of the venous system and 

 rise of vena cava pressure produced by general muscular spasm. On 

 the other hand, the writer has measured the intracranial tension 

 in a certain patient when standing upright, and found it to be slightly 

 below zero. This patient had been trephined for symptoms of lunacy. 

 Men in the spasms of strychnine poisoning are conscious, and the above 

 patient was also conscious. It is clear, therefore, that the functions 

 of the brain can continue at circulatory pressures varying from zero 

 to 50 mm. Hg. 



No evidence has been found of the existence of cerebral vasomotor 

 nerves, either by means of stimulation of the vasomotor centre, or 

 of the central end of the spinal cord, after division in the upper 

 dorsal region ; or by stimulation of the stellate ganglion. That is to 

 say, the whole of the cranial nerve supply, and the whole of the 

 sympathetic supply to the carotid and vertebral arteries, have been 

 stimulated, and the result has been negative. 3 



Nor is there any anatomical evidence of the existence of a local 

 vasomotor mechanism. Gulland has entirely failed, by every known 

 histological method, to demonstrate vasomotor nerves in the vessels 

 of the pia mater. 4 



In every experimental condition the cerebral circulation passively 

 follows the changes in the general arterial and venous pressures. The 

 intracranial or cerebral venous pressure varies directly and absolutely 

 with vena cava pressure, but only proportionately with aortic pressure. 

 If both alter, the cerebral venous or intracranial pressure is affected by 

 both, and an alteration of pressure of equal amount, and in opposite 

 directions in both, will affect the intracranial or cerebral venous pressure 

 from the venous side to a far greater degree than from the arterial side, 

 because it is on the arterial side that the peripheral resistance lies. If 

 the torcular Herophili be opened in the freshly-killed animal, and the 



1 Grashey, Fc-stschr. f. L. A. JBicchner, Munchen, 1892, S. 61 ; Geigel, "Die Mechanik 

 der Blutversorgung des Gehirns," Stuttgart, 1890 : Vlrchow's Archiv, 1890-91, Bd. cxix. 

 S. 93 ; cxxi. S. 432 ; cxxiii. S. 27 ; cxxv. S. 92. 



2 Lewy, ibid., 1890, Bd. cxxii. S. 146. 



3 Hill, "Cerebral Circulation," p. 42. 



4 i/burn. Physiol., Cambridge and London, 1895, vol. xviii. p. 361, 



