278 THE CIRCULATION IN THE BLOOD-VESSELS. [CH. xxi. 



capable of change of size ; and composed, as they are, of the 

 tough tissue of the dura mater, and, in some instances, bounded 

 on one side by the bony cranium, they are not compressible by 

 any force which the fulness of the arteries might exercise through 

 the substance of the brain ; nor do they admit of distension when 

 the flow of venous blood from the brain is obstructed. No valves 

 are placed between the vertebral veins and the vena cava ; the 

 vertebral veins anastomose with the cerebral sinuses. Hence on 

 squeezing the thorax and abdomen, venous blood can be pressed 

 from those parts out of any opening made into the longitudinal 

 sinus. Expiration acts in the same way ; it raises the cerebral 

 venous pressure; if the skull wall is defective the brain expands 

 owing to the distension of its capillaries during the expiratory act. 

 The exposed brain also expands with each systole of the heart. 

 Owing to the fact that the brain lies enclosed in the cranium, 

 the arterial pulse is transmitted through the brain substance to 

 the cerebral veins and so the blood issues from these in pulses. 



Since the brain is enclosed in the rigid cranium the volume of 

 blood in the cerebral vessels cannot alter unless the volume of 

 the other cranial contents alter in the opposite sense. 



These conditions of the brain and skull led Monro and Kellie 

 many years ago to advance the opinion that the quantity of blood 

 in the brain must be the same at all times. This doctrine has 

 been frequently disputed, and many have advanced the theory 

 that increase or diminution of the blood is accompanied with 

 simultaneous diminution or increase of the cerebro-spinal fluid, 

 so that the contents of the cranium are kept uniform in volume. 

 But the recent work of Leonard Hill* has shown that the Monro- 

 Kellie doctrine is true. Histological evidence has recently been 

 obtained of the existence of nerve plexuses round the pial arteries. 

 The arteries are muscular, and the nerves therefore are most 

 probably vaso-motor in function. Experimental evidence so far, 

 however, has not established the action of these nerves ; the 

 cerebral circulation passively follows the slightest changes in 

 aortic and, more especially, vena cava pressure, and no active 

 vaso-motor change has been conclusively proved. The velocity 

 of blood-flow through the brain is thus influenced markedly by 

 the condition of the vessels of the splanchnic area. If the tone 

 of the skeletal muscles and that of the vessels be suddenly 

 inhibited by fear, or temporarily destroyed by shock, the blood 

 will drop owing to its weight into the dilated and supported 



* I am much indebted to Mr. Hill for assistance in the preparation of 

 these paragraphs on the cerebral circulation. 



