830 THE VENOUS SINUSES. [BOOK in. 



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 

 ( 330) ; 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 pres- 

 sure ; and we may take it for granted, unless otherwise indicated, 

 that greater or less pressure is due to more or less blood pass- 

 ing 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 increase of sup- 

 ply 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 teach- 

 ings of the lever must be interpreted by help of a simultaneous 

 observation of the general arterial pressure and of the blood- 

 pressure in the veins of the neck ; or the pressure in the sinuses 

 themselves may be measured by introducing a cannula directly 

 into the torcular Herophili. Moreover, the argument which 

 we used ( 337) in reference to the kidney may be applied 

 here and probably with equal force, namely, that the value of 

 the blood stream for the nutrition of the tissue is dependent 

 not alone on the amount of blood-pressure, but also and espe- 



