THE CEREBRAL CIRCULATION. 145 



The essential point which is required in all experiments on the cerebral 

 circulation is, that the cranial cavity should be a closed cavity, as in the 

 normal condition. 



Changes in intracranial tension cannot be registered by screwing an 

 ordinary manometer x into the skull, because, with any increase of tension, 

 the brain floats up against the trephine hole, and acts towards it as a valve, 

 so that no more fluid can escape into the manometer. The cranio-vertebral 

 cavity cannot be put in connection with a tambour, or used as an oncometer, 2 

 because the brain on its expansion closes up, not only a trephine hole in the 

 roof of the cranium, but the foramen magnum in the base. Moreover, the 

 cerebro-spinal fluid leaks from the cavity. 



Of considerably greater value, as a method, is the record of the outflow of 

 blood, per second, from the lateral sinuses. 3 Hiirthle and Cavazzani have 

 recorded the arterial pressure in the circle of Willis 4 by placing a cannula in 

 the peripheral end of the carotid artery, after tying all but the cerebral branch. 



Cybulski, by means of his instrument, the photohaematochometer, has 

 investigated the rate of flow in the internal carotid artery under varying con- 

 ditions of experiment. 5 



Koy and Sherrington, 6 by an ingenious plethysmographic method, recorded 

 the variations of the vertical diameter of the brain with the cranium open. 

 The conditions here are not normal. In the closed cranium, large alterations 

 of blood volume are impossible. In the open cranium, with the atmospheric 

 pressure brought to bear on the cerebral capillaries, the physical conditions 

 of the cerebral circulation are altered. 



Hill and Bayliss 7 have simultaneously recorded the aortic pressure, the 

 vena cava pressure, the intracranial pressure, and the cerebral venous pressure ; 

 the cranium being, as in the normal condition, a closed cavity. The intra- 

 cranial pressure is measured by means of the brain-pressure gauge, while the 

 cerebral venous pressure is obtained by placing a cannula in the torcular 

 Herophili. The pressure of the cerebro-spinal fluid can be measured by 

 trephining the atlas, opening the dura mater, screwing a tube into the 

 trephine hole, and connecting this tube with a water manometer. 



By these means, the following principles of the cerebral circulation 

 have been determined. 



When the aortic pressure rises, the expansion of cerebral volume 

 can take place only to a certain limited amount, for, as soon as all the 

 cerebro-spinal fluid has been driven out from the cranium, the brain is 

 everywhere in contact with the rigid wall of the skull. Any further 

 expansion of the arteries can only take place by an equivalent com- 

 pression of veins, for the semifluid brain matter is incompressible. 

 The reservoirs of blood in the veins will therefore be so far constricted, 

 until the cerebral venous pressure rises to the pressure of the brain 

 against these veins. Thus, as the arterial tension rises, the whole 



1 Salathe\ "Trav. du labor, de Marey," 1876, tome ii. p. 362. 



2 Falkenheim and Naunyn, Arch. f. exper. Path. u. Pharmakol., Leipzig, 1887, Bd. 

 xxii. S. 261 ; Dean, Joum. Path, and Bacteriol., Edin. and London, 1892, vol. i. p. 26 ; 

 Knoll, Sitzungsb. d. k. Akad. d. Wissensch., math.-naturw. CI., Wien, 1886 (3), Bd. xciii. 

 S. 227. 



3 Gartner and Wagner, Wien. med. Wchnschr., 1S97, S. 602, 639. 



4 Hiirthle, Arch. f. d. ges. Physiol., Bonn, 1889, Bd. xliv. S. 582; Cavazzani, Arch, 

 ital. de biol., Turin, vol. xvi. p. 23; vol. xix. p. 24; Centralbl. f. Physiol., Leipzig u. 

 Wien, 1896. 



5 Cybulski, "0 ucisku mozgu," Krakow, 1891 ; Centralbl. f. Physiol., Leipzig u. Wien, 

 1890, S. 834. 



6 Roy and Sherrington, Joum. Physiol., Cambridge and London, 1890, vol. xi. p. 85. 



7 Bayliss and Hill, ibid., 1895, vol. xviii. p. 334; and Hill, "Cerebral Circulation," 

 p. 40. 



VOL. II. — IO 



