262 THE CIRCULATION OF THE BLOOD 



Vasomotor Nerves. These possibilities raise the question as to whether 

 there are functionally active vasomotor nerves to the cerebral vessels. 

 Histologists have definitely demonstrated nerve fibers running on to the 

 cerebral vessels, especially by the use of the intra vitam methylene blue 

 method of staining (Huber, Hunter, etc.), but this does not of course 

 necessarily indicate that the fibers normally cause the arterial walls to 

 expand and contract. The only basis upon which such a claim could be 

 put forth is an actual demonstration of changes in intracranial blood- 

 flow occurring independently of changes in systemic arterial or venous 

 pressures. 



Leonard Hill and Bayliss, and later Leonard Hill and Macleod, 20 have 

 most diligently sought for such evidence, but with entirely negative 

 results. Kecords were taken of the intracranial pressure, the cerebral 

 venous pressure and the pressure in the circle of Willis (by a cannula 

 inserted in the peripheral end of the internal carotid artery), as well 

 as the arterial and venous pressures in the systemic vessels carotid and 

 jugular). Since any vasomotor fibers must presumably be derived from 

 the vasomotor centers, and since these fibers must gain the cerebral 

 vessels through the stellate ganglion and ultimately travel into the 

 cranial cavity along the outer coats of the arteries, the above pressures 

 were simultaneously observed before and during electrical stimulation 

 at these places. It was found that any change that did occur could in- 

 variably be attributed to changes in the circulation as a whole ; there 

 was never any alteration in pressure locally in the brain for which the 

 occurrence of local constriction or dilatation of the vessels had to be 

 assumed. 



Other observers have attempted to investigate the problem by meas- 

 urement of the volume of blood leaving the brain, but with similarly 

 negative results. 



But an objection can be raised to these experiments on the ground that there 

 might be feebly acting vasomotor influences, the effect of which would become entirely 

 masked by the much more potent influence exerted on the bloodflow by changes in the 

 circulation as a whole. As pointed out by Wiggers, the only way by which local 

 changes in the bloodflow through the intracranial vessels can be expected to reveal 

 themselves is by measuring the entire outfloAV, a measurement which, however, it is im- 

 possible to make in an intact animal on account of the many patlnvays through which 

 the venous blood can leave the skull. Measurement of the outflow by one of them 

 does not by any means indicate the magnitude of total outflow. To overcome these 

 difficulties, Wiggers proceeded to measure the outflow from all the cranial vessels of 

 oxygenated Locke's solution perfused into the cerebral artries under constant pressure. 

 It was found that the otherwise constant rate of outflow became decidedly curtailed 

 when adrenalin was added to the Locke 's solution. If we assume that this drug acts 

 only on arterial. muscle having functionally active vasoconstrictor nerves, then the result 

 would prove the presence of such fibers to the cerebral vessels, but even granted this, 



