240 A TEXTBOOK OF PHYSIOLOGY 



To effect this the intracranial pressure is measured by means of 

 a brain-pressure gauge (Fig. 126), while the cerebral venous pressure 

 is obtained by screwing a cannula into the torcular Herophili, a bony 

 cavity within the occiput of the dog. 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 of the cranium, the brain is 

 everj'where 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 semi-fluid brain matter is incompressible. 

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

 stricted, until the cerebral venous pressure rises to the prossure of 

 the brain against these veins. Thus, as the arterial pressure lises, the 

 whole circulatory- system of the brain will assimilate itself more and 



F" 



B 



FIG. 126. HILL'S CEREBRAL PRESSURE GAUGE. 



more to a scheme of rigid tubes. Thus 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. 



The intracranial pressure is in all physiological conditions the same 

 as the cerebral venous pressure. The intracranial pressure, or pres- 

 sure of brain against skull wall, is, in fact, that pressure which remains 

 after the force of the heart has been expended in driving the blood 

 through the cerebral arterioles. It is therefore an ever-varying 

 quantity. 



In the normal conditions, with the animal in the horizontal posi- 

 tion, the intracranial pressure may approximate to about 100 mm. HgO. 



In the spasms of strychnine-poisoning the intracranial tension 

 may rise to 40 to 50 mm. Hg. Th's is due, not only to the rise of 

 arterial pressure, but to the rise of vena cava pressure produced by 

 general muscular spasm. 



The intracranial or cerebral venous pressure varies directly with 

 changes in vena cava pressure, but only proportionately with those in 

 aortic pressure. If the torcular Herophili be opened in a freshly 

 killed animal, and the abdominal veins be compressed, venous blood 

 can be driven out of the torcular in a continuous stream. Between 



