THE BRAIN. 501 



considerable anastomosis of vessels Ln 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 ex- 

 plained 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 

 a normal condition. Though the quantity of bipod 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 vascular 

 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 have been removed, so that the brain was visible, 

 show that during sleep the blood-vessels are much less promi- 

 nent than usual ; and it is well known that means calculated to 

 diminish the circulation in the brain, as cold and pressure, favor 

 sleep. It is also well established by general experience that 

 withdrawal of the usual afferent impulses through the various 

 senses favors sleep. A remarkable case is on record of a youth 

 whose avenues for sensory impressions were limited to one eye 

 and a single ear, and who could be sent to sleep by closing 

 these against the outer world. . Yet this subject after a long 

 sleep would awake of his own accord, showing that, while affer- 

 ent impulses have undoubtedly much to do with maintaining 

 the activity of the cerebral centers, yet their automaticity (in- 

 dependence) must also be recognized. 



