CIRCULATION. 495 



of the brain and those of other organs was recognized in part at least by 

 the younger Monro as long ago as 1783. Monro declared that the quantity 

 of blood within the cranium is almost invariable, " for, being enclosed in a 

 case of bone, the blood must be continually flowing out of the veins that room 

 may be given to the blood which is entering by the arteries, as the substance 

 of the brain, like that of the other solids of our body, is nearly incompress- 

 ible." Further differences between the circulation in the brain and in other 

 organs are introduced by the presence of the cerebro-spinal fluid in the ventri- 

 cles and in the arachnoidal spaces at the base of the brain. This fluid may pass 

 out into the spinal canal and thus leave room for an increase in the amount 

 of blood in the cranium. Finally, a rise of pressure in the arteries too great 

 to be compensated by the outflow of cerebro-spinal fluid may lead to com- 

 pression of the venous sinuses and a decided change in the relative distri- 

 bution of the blood in the arteries, capillaries and veins conditions which are 

 not present in extracranial tissues. It is evident, therefore, that the methods 

 employed in the search for vaso-motor nerves within the cranium must take 

 into account many sources of error that are absent in vaso-motor studies of 

 other regions. It is, indeed, probable that incompleteness of method will go 

 far toward explaining the disagreement of authors as to the presence of vaso- 

 motor nerves in the brain. According to Bayliss and Hill, 1 the most recent 

 investigators of this subject, it is necessary to record simultaneously the arterial 

 pressure, the general venous pressure, the intracranial pressure and the cerebral 

 venous pressure, the cranium as in the normal condition being kept a closed 

 cavity. In their experiments, " a cannula was placed in the central end of the 

 carotid artery. A second long cannula was passed down the external jugular 

 vein, and on the same side, into the right auricle. The torcular Herophili was 

 trephined, and a third cannula, this time of brass, was screwed into the hole 

 thus made." The intracranial pressure was recorded by a cannula connected 

 through another trephine-hole with the subdural space. 



Bayliss and Hill could find no evidence of the existence of cerebral vaso- 

 motor nerves. The cerebral circulation, according to them, passively follows 

 the changes in the general arterial and venous pressure. Gulland 2 has examined 

 the cerebral vessels by the Golgi, Ehrlich, and other methods, to determine 

 whether nerve-fibres could be demonstrated in them. None were found. It 

 is probable that the blood-supply to the brain is regulated through the bulbar 

 vaso-constrictor centre. 3 Anaemia or asphyxia of the brain stimulates the cells 

 composing this centre, vascular constriction of many vessels follows, and more 

 blood enters the cranial cavity. The vessels of the splanchnic area play a 

 chief part in this regulative process. 4 Their importance to the circulation in 

 the brain is shown by the fatal effect of the section of the splanchnic nerves 

 in the rabbit. On placing the animal on its feet r so much blood flows 

 into the relaxed abdominal vessels that death may follow from anaemia of the 

 brain. 



1 Bayliss and Hill, 1895, p. 837. 2 Gulland, 1895, p. 361. 



3 Bayliss and Hill, 1895, p. 358. 4 Wertheirner, 1893, p. 297. 



