THE VASCULAK ARRANGEMENTS OF THE BRAIN. 735 



weight of bloocUin the brain at anyone time amounts to about five per cent, 

 of the total weight of the organ, being about the same as in the muscles, in 

 the kidney it amounts to nearly twelve per cent., and in the liver to as much 

 as nearly thirty per cent. Making every allowance for the relative small 

 size and functional importance of the rabbit's brain, the blood-supply of 

 even the human brain must still be small ; and making every allowance for 

 rapidity of current, the interchange between the blood and the nervous ele- 

 ments must also be small. In other words, the metabolism of the brain-sub- 

 stance is of importance not so much on account of its quantity as of its special 

 qualities. 



The circulation in the brain may be studied by help of various methods. 

 A manometer may be connected with the peripheral end of the divided inter- 

 nal carotid artery, a second manometer being attached in the usual way to 

 the central portion. Since the peripheral manometer records the blood-pres- 

 sure in the circle of Willis transmitted along the peripheral portion of the 

 carotid artery, variations of pressure in the circle of Willis may thus be 

 studied ; and a comparison of the peripheral with the central manometer 

 will indicate what general changes are taking place in the circulation 

 through the brain. Thus a fall of pressure in the peripheral manometer 

 unaccompanied by any corresponding fall in the central manometer would 

 show that the " peripheral resistance " in the brain was being lowered, in 

 other words, that the vessels were being dilated. 



In another method, in the dog, the outflow of venous blood from the 

 lateral sinus through the posterior facial vein has been measured. The 

 freedom with which blood passes along the sinuses justifies the assumption 

 that the outflow through the open vein gives an approximate measure of the 

 rate of flow under natural conditions ; still the results are only approximate, 

 and besides, the continued loss of blood introduces error. 



A third method is a plethysmographic one. The skull is made. to serve 

 as the box of the plethysmograph or oncometer ( 346) ; a small piece of 

 the roof having been removed by the trephine, a membrane is fitted to the 

 hole, and the movements of the membrane are recorded by help of a piston 

 and lever or directly by a lever. In young subjects, the fontanelle, or por- 

 tion of the cranium not yet ossified, may be utilized as a natural membrane, 

 and its movements recorded in a similar manner. When the instrument is 

 fitted to the hole in a water-tight manner, this method records variations in 

 internal pressure; and we may take it for granted, unless otherwise indi- 

 cated, that greater or less pressure is due to more or less blood passing to 

 the brain. But the amount of pressure brought to bear on the recording 

 instrument will also depend on the readiness with which the cerebro-spinal 

 fluid escapes from the cavity of the skull ; if there be a hindrance to the 

 escape, or, on the other hand, an increased facility of escape, the same in- 

 crease of supply of blood will produce in one case a less, in the other a 

 greater movement of the lever. If the membrane be attached loosely to 

 the hole so as to allow free escape of the cerebro-spinal fluid, the lever 

 practically resting on the surface of the cerebral hemisphere, the method 

 records variations in the dorso-ventral diameter of the hemisphere, and 

 these may be taken as measuring variations in the volume of the brain and 

 so in the blood-supply. In neither form, however, does the method by itself 

 give us all the information which we want. An increase of blood in the 

 brain, and therefore an expansion of the brain, and so a movement of the 

 recording instrument, may result either from a fuller arterial supply or 

 from hindrance to the venous outflow ; the former condition is, at least in 

 most cases, favorable to, the latter always and distinctly injurious to, the 

 activity of the nervous structures ; hence the teachings of the lever must 



