248 



The venous blood is collected by the small, very thin-walled and valve- 

 less cerebral veins. These run together to form larger veins dis- 

 charging into the sinuses, the openings into which are kept patent by 

 the arrangement of dura mater around the orifices. The sinuses exist 

 between the dura and skull and are so constructed that they can not 

 be compressed, particularly those at the base of the brain. From them 

 the blood is conveyed mainly to the internal jugular vein, some of it 

 however escaping by the anastomoses existing between the cavernous 

 sinus and the opththalmic veins, and by the venous plexus of the spinal 

 cord. The most striking peculiarities of the veins are their patulous con- 

 dition and the absence of valves, so that any change in the blood pres- 

 sure in the internal jugular vein must be immediately reflected in that of 

 the venous sinuses. This explains why compression of the abdomen 



Fig. 79. Schema to show the relations of the Pacchionian bodies to the sinuses: d, d, Folds 

 of the dura mater, inclosing a sinus between them; v.b., the blood in the sinus; a, the arachnoidal 

 membrane; p, the pia mater; Pa., the Pacchionian body as a projection of the arachnoid into the 

 blood sinus. (From Howell's Physiology.) 



causes venous blood to flow from an opening made in the longitudinal 

 sinus. 



In considering the cerebral circulation, another factor that must be 

 borne in mind is the presence of cerebrospinal fluid. This is contained 

 in the subarachnoid spaces of the brain and spinal cord, these spaces, in 

 the case of the brain, being often considerably enlarged to form the 

 cisternse. The cerebrospinal fluid is also present in the ventricles of the 

 brain, which it will be remembered communicate with the subarachnoid 

 spaces through the foramen of Magendie, etc. It is unlikely that the 

 cerebrospinal fluid is of much importance in connection with the control 

 of the blood supply to the brain tissue. It may be merely a lubricating 

 fluid; at least it is so small in amount (60 to 80 e.c. in man) as to be 

 apparently of little value in bringing about an alteration in brain volume. 



