144 THE MECHANISM OF THE CIRCULATION. 



cerebral pulsation is least when the intracranial tension is normal. 

 In the living animal, the chief contents of the cranial cavity, beside 

 the brain substance, is blood, and not cerebro-spinal fluid. The living 

 brain, with its circulating blood, almost entirely fills the cranium, and 

 the fluid that moistens its surfaces is little more in amount than the 

 synovial fluid in a joint. The cerebral subarachnoid space is chiefly a 

 potential rather than an actual space. The rate of transudation of the 

 cerebro-spinal fluid seems to depend directly on the difference between 

 the cerebral venous and the subarachnoid pressure. 



In 1783, Alexander Monro the younger 1 put forward the view that 

 the quantity of blood within the cranium is almost invariable. " For, 

 being enclosed in a case of bone," he writes, " the blood must be 

 continually flowing out of the veins, that room may be given to the 

 blood which is entering by the arteries. For, as the substance of the 

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

 the quantity of blood within the head must be the same, or nearly the 

 same, at all times, whether in health or disease, in life, or after death, 

 those cases only excepted in which water or other matter is effused or 

 secreted from the blood vessels ; for, in these, a quantity of blood equal 

 in bulk to the effused matter will be pressed out of the cranium." 



Monro's conjecture is confirmed by experiment. If a glass plate be 

 screwed into a trephine hole, on compressing the innominate and sub- 

 clavian arteries, the pial vessels can be seen to become less in size. 

 The brain, however, does not collapse or retreat from the glass window. 

 If, on the other hand, the glass window be faultily placed, and allow 

 leakage into the cranial cavity, air passes within, and the brain collapses 

 under atmospheric pressure. This experiment proves that the brain in 

 the closed cranium can by no means completely empty itself of blood, 

 even though the arterial pressure should fall to zero. 



The brain can only empty itself of blood in so far as the cerebro- 

 spinal fluid and this is small in amount can take the place of the 

 blood. If an animal be placed in the vertical feet-down position, and 

 the skull be trephined, then, on opening the dura mater, the brain, 

 which before was in close apposition with that membrane, can be seen 

 collapsing, as it is emptied of blood by atmospheric pressure. 



It is evident that in all physiological conditions the blood content 

 of the brain can vary suddenly only to a slight degree by the ebb and 

 flow of the cerebro-spinal fluid, and that Monro's doctrine is, to all 

 intents and purposes, true. No sure evidence of the condition of the 

 cerebral circulation can be drawn from examination of the brain after 

 death, for in many different ways the relative volume of blood and 

 of serous fluid within the cranium may be altered by post-mortem 

 changes. 



The following methods have been employed in investigations on the 

 cerebral circulation : 



1. Direct observation of the pia mater. 



2. A record of the volume of the brain. 



3. A measurement of intracranial pressure. 



4. Measurement of the blood pressure in the cerebral vessels. 



5. Measurement of the velocity of blood flow in the cerebral vessels. 



1 <c Observations on the Structure and Functions of the Nervous System," Edin., 1783. 

 Cf. Abercrombie, Edin. Med. and Sury. Journ., 1818, vol. xiv. p. 553 ; "Pathological and 

 Practical Researches on Diseases of the Brain," etc., 1828. 



