526 ANIMAL PHYSIOLOGY. 
efflux of the cerebro-spinal fluid. The part played by this ar- 
rangement has, however, been probably overestimated. But 
the peculiar venous sinuses do, it is likely, serve to regulate 
the blood-supply; being very large, they may answer as tem- 
porary overflow receptacles. An inspection of the fontanelles 
of an infant reveals a beating corresponding with the pulse; 
and, when a large part of the cranium is removed in an animal, 
a plethysmograph shows a rise in volume corresponding with 
the pulse and the respiratory movements, as in the case of the 
fontanelles. But, besides these, periodic waves of contraction 
are now known to pass over the cerebral arteries. 
Whether the latter is part of a general wave traversing the 
whole arterial system is as yet uncertain. Though there is 
considerable anastomosis of vessels in the encephalon, it is not 
equal to what takes place in many other organs. It is well 
known that a clot or other plug within a cerebral vessel is 
more serious than in many other regions, which is partly to be 
explained by the lack of sufficient anastomosis for the vascular 
needs of the parts. It is also well known that, in organs which 
constitute parts of a related series, as the different divisions of 
the alimentary tract, all are not usually at the same time vas- 
cular to the same extent. While they act functionally in rela- 
tion to each other, they exemplify also a certain degree of inde- 
pendence. Such a condition of things is now known to exist in 
the brain—i. e., certain areas may be abundantly supplied with 
blood as compared with others; and it seems highly probable 
that a condition of equal arterial tension throughout is scarcely 
anormal condition. Though the quantity of blood contained 
within the vessels of the whole brain at any one time is not so 
large as in some other organs (glands), yet the foregoing facts 
and the rapidity of the flow must be taken into account. The 
capillaries are very close and abundant, in the gray matter es- 
pecially; and it is to be borne in mind that it is chiefly these 
vessels which are concerned in the actual metabolism (nutri- 
tion) of parts. However, the chemical changes in the nervous 
system being feeble, it would appear probable that it does its 
work with less consumption of pabulum than other parts of 
the body. We wish to lay stress on the local nature of vas- 
cular dilatation in the brain as, it greatly assists in explaining 
certain phenomena about to be considered. 
Sleep.—Observations upon animals from which portions of 
the cranium had been removed, so that the brain was visible, 
show that during sleep the blood-vessels are much less promi- 
