PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 259 



the expiratory rise in venous pressure which is well known to occur in 

 the former to be directly transmitted to the brain. 



This brings us to the second part of our first question : What determines 

 the intracranial pressure ? To answer it we must know something of the 

 method by which the pressure is measured. This has been most successfully 

 done by Leonard Hill, 19 who devised an instrument called the cerebral 

 pressure gauge, consisting of a brass tube closed at one end by rubber 

 membrane and screwed into a trephine hole. The outer end of the tube 

 is joined to a narrow glass tube connected with a pressure bottle. The 

 whole system is filled with fluid except for a minute bubble of air in the 

 narrow glass tube. Any changes in pressure in the brain cause correspond- 

 ing movements of the bubble, and the magnitude of the change is meas- 

 ured by readjusting the pressure bottle so as to bring the bubble back to 

 its original level. It has been found that the pressure may vary from 

 zero to 50 mm. Hg. (as in strychnine convulsions), and that these variations 

 depend entirely on circulatory conditions, there being no compensatory 

 mechanism by which the pressure is kept constant. The average pressure 

 under physiological conditions is 100-130 mm. H 2 0. 



The intracranial pressure varies directly with the venous pressure within 

 the skull, and. it passively follows changes in the pressures in the arteries 

 and veins of the systemic circulation. This implies that the efficiency of 

 the cerebral circulation will be dependent very largely upon alterations 

 in the capacity of the splanchnic area, the greatest reservoir of blood in 

 the body. By actual measurement it has also been found that: 



1. The pressure within the lateral sinuses of the brain (measured by 

 connecting a tube and manometer with the torcular Herophili) varies 

 absolutely with the intracranial pressure. It therefore exhibits pulsa- 

 tions which mirror precisely those observed in the cerebral pressure 

 gauge. 



2. Both these pressures passively follow changes in the pressure in 

 the right auricle. They also run more or less parallel with changes in 

 arterial pressure, and there is never any change in either of them which 

 can not be traced to some general circulatory condition. 



A few of the many experiments performed by Leonard Hill and others will serve 

 to prove these far-reaching conclusions: 



1. In asphyxia caused by cessation of the respiratory movements in a curarized 

 animal, the cerebral venous pressure at first falls with the fall in systemic pressure 

 and then rises as the arterial hypertension sets in. In the last stage, however, al- 

 though the arterial pressure is quickly falling, the venous pressure rises and with 

 it the cerebral venous pressure. During vagus inhibition the fall in arterial pressure is 

 so marked that the intracranial pressures fall at first and only rise later, correspond- 

 ing to the rise in venous pressure (Fig 80). 



2. During administration of ether, alterations in cerebral pressure become marked 

 only when there is extensive muscular movement or hyperpnea. Chloroform, on the 



