96 A MANUAL OF VETERINARY PHYSIOLOGY 



arterial anastomoses — for example, the Circle of Willis and 

 the Rete Mirabile of ruminants, which insures that not only 

 does the blood enter with diminished velocity, but that if a 

 temporary obstruction occurs in one vessel its work is readily 

 performed by the others. The rete mirabile alluded to, which 

 forms the arterial plexus on the base of the brain of ruminants, 

 is considered by some to regulate the flow of blood to the brain 

 when the head is depressed during grazing, and, it is said, 

 accounts for the absence of cerebral haemorrhage in these animals. 

 It is probable that this may be one of its functions, but the 

 horse possesses no rete, and his head is depressed during grazing 

 for more hours out of the twenty-four than is the case with 

 ruminants. It has probably, therefore, some other function to 

 perform. 



The free anastomoses of the vessels of the brain guarantee to 

 it an uninterrupted blood-supply. In the dog both internal 

 carotids and both vertebrals have been ligatured without causing 

 unconsciousness or death, the supply of blood being kept up 

 through the anterior spinal artery. The veins of the brain 

 empty their contents into venous sinuses which from their struc- 

 ture are well protected from compression. There are no valves 

 to the openings into the sinuses, nor are there valves in the 

 thin-walled small veins of the brain. 



The circulation in the brain is peculiar, inasmuch as its sub- 

 stance is contained within a closed unyielding cavity. A small 

 space formed by the membranes exists between the skull and 

 the brain, and in this is found a little fluid, which may pass into 

 the interior of the brain or backward into the spinal canal. The 

 cerebral fluid acts as a water-pad, and this fluid may be rapidly 

 absorbed, for it readily passes into the veins at any pressure 

 higher than that in the venous circulation. Nevertheless, the 

 space which this renders available is very small, so that the 

 brain may be regarded as incompressible. Special provision 

 must, therefore, exist in its circulation to guard against a rise 

 in blood-pressure, and this is furnished by dilatation of any one 

 set of vessels producing a constriction of the other set. By this 

 means no increase in size nor increase in blood capacity occurs ; 

 all that is affected is the velocity of flow. If the arterial pressure 

 of the brain rises and that in the veins remains constant, there is 

 an increased velocity of flow ; if the arterial pressure is constant 

 and the venous pressure rises, there, is a diminished velocity of 

 flow. 



The brain presses against the cranium with a pressure equal 

 to that in the capillaries. The brain-pressure, cerebro- venous 

 pressure, and cerebro-spinal fluid pressure have been shown by 

 Hill to be one and the same. 



