326 PRESSURE ON THE BRAIN. 



internal carotid and two vertebral arteries. The impetus of 



Supply of .11111- 



blood "to the the current is checked Toy the sinuous course these vessels 

 brain - take, or "by their breaking promptly into capillary branches. 



A freedom of anastomosis among them, as is well displayed in the circle 

 of Willis, affords abundant provision for accidental stoppages or re- 

 straints. 



Although the brain is inclosed in an unyielding cavity, it is subject to 

 Atmos heric ^ pressure of the air, a fact which, though it has been de- 

 pressure on the nied by some physiologists, follows from ordinary physical 

 principles. And since the quantity of blood present at any 

 moment in the organ varies with the contemporaneous functional activ- 

 ity, being greater as that activity is greater, the cerebro-spinal fluid also 

 varies in amount. Through this fluid an equality of pressure is there- 

 fore insured, no matter what may be the quantity of blood in the brain. 



The cerebro-spinal fluid, the quantity of which has been estimated at 

 Cerebro-spinal two ounces, is readily absorbed and as readily reproduced. 

 fluid - The act of adjustment between it and the blood requiring a 



certain period for its completion, the brain can not instantaneously be 

 brought to its maximum action. Thus, as all persons observe, when we 

 undertake any unusual intellectual duty, there is a certain preparatory 

 period to be passed through, as the common expression is, " for compos- 

 ing the thoughts." t 



Pressure upon the brain, either applied mechanically or through acci- 

 Effect of me- dental effusions, produces at once functional inactivity, prob- 



chanicai pres- a ^j ty interference with the due circulation of the blood ; 

 changes in the and, in like manner, any marked change in the chemical re- 

 blood. lations of that fluid exerts on the brain a corresponding ef- 



fect. Thus, when oxygen gas is breathed, or, still better, protoxide of 

 nitrogen, which is more soluble in the blood, the processes of intellection 

 go on in an exaggerated way, and ideas in rapid succession, and in unu- 

 sual forms of combination, flit through the mind ; but, as the consequence 

 of this, since the lungs can not remove with the necessary promptness 

 the carbonic acid which is arising, the narcotic effects of that body are 

 soon experienced ; and this is also the case in alcoholic intoxication, in 

 the advanced stages of which the accumulation of carbonic acid in the 

 blood gives rise to the same result. 



That different regions of the brain have independent though mutually 

 Effect of size commissured faculties, is fully established by the phenomena 

 tion onhe" f the nery es of sense, nor can there be any doubt that these 

 brain. differences of physiological function are directly dependent on 



differences of anatomical structure. It is, indeed, to structural differences 

 that we should impute the greater or less efficiency of the whole organ, 

 as much as to differences of its weight. Because of a higher elaboration, 



