CHAP, ii.] THE BRAIN. 1135 



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 shew 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 ( 410) ; 

 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 portion 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 indicated, 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 in- 

 strument 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 increase 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, 

 favourable to, the latter always and distinctly injurious to, the 

 activity of the nervous structures ; hence the teachings of the 

 lever must be corrected by a simultaneous observation of the 

 general arterial pressure and of the blood-pressure in the veins of 

 the neck. Moreover, the argument which we used ( 417) in 

 reference to the kidney may be applied here and probably with 

 equal force, namely, that the value of the blood stream for the 



F. 72 



