442 THE NERVOUS REGULATION OF THE BLOOD-VESSELS 



more, the connection between the lymphatic spaces of the cerebrum 

 and the veins is sufficiently free to allow an escape of this liquid when- 

 ever the intracranial pressure suffers a more marked and lasting 

 increase. 



Under normal conditions, the pressure of the cerebral fluid pre- 

 serves a direct relationship to the pressure of the brain, but only witHin 

 certain limits. Thus, while a certain compensation is possible, its 

 range is limited. For this reason, a decided increase in the intracranial 

 pressure must invariably be followed by a rise in the venous pressure 

 in consequence of the compression of the veins. This change in turn 

 leads to a rise in the arterial pressure, because the influx of the arterial 

 blood is thereby retarded. In the same way, a marked increase in 

 the venous pl-essure, or in the general cerebral blood pressure, must 

 be followed by an elevation of the intracranial pressure, because the 

 spaces containing the cerebral fluid, are thereby compressed while the 

 escape of the latter into the veins is made impossible. Conditions 

 of this kind may be produced without much difficulty by various 

 experimental procedures. They are also associated with different 

 pathological processes such as tumors, extravasations of blood, an 

 excessive production of cerebrospinal liquid, and others. In all these 

 cases the intracranial pressure is raised beyond the limits of compen- 

 sation so that a compression of the brain results which in turn is fol- 

 lowed by far reaching and grave functional disturbances. Hindrances 

 to the venous return, or a greater inrush of arterial blood, produce the 

 same general effects, the only difference between them being that, in 

 the latter instance, the cerebral blood pressure is affected first and the 

 intracranial pressure last. 



The intracranial pressure may be increased by inserting a trephine 

 cannula in the region of the parietal lobe which is connected with a 

 reservoir containing warmed saline solution. The dura should be 

 incised, because this membrane is so tense that it protects the men- 

 inges against any pressure which may be produced by raising this 

 reservoir. It will be found that the general blood pressure rises very 

 abruptly as soon as the intracranial pressure has exceeded that of the 

 blood. It does not remain at this high level, however, because the 

 heart soon displays a decided diastolic tendency and ceases to beat 

 altogether if the compression is continued for too long a time. These 

 changes are associated with an inhibition of respiration. Two ex- 

 planations have been offered for this phenomenon. Adamkiewicz has 

 stated that it is occasioned by the mechanical damming back of the 

 arterial blood in front of the cranial orifices, while Gushing^ believes 

 that it arises in consequence of a reflex vasomotor reaction. By meas- 

 uring the blood flow through the carotid artery with the help of a 

 stromuhr, Burton-Opitz and Edwards ^ have shown that the brain 



1 Am. Jour: of Med. Sciences, 1902 and 1903; also see: Eyster, Burrows and 

 Essick, Jour. Exp. Med., xi, 1909, 489. 

 ^ Wiener klin. Wochenschr., 1916. 



