PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 255 



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 

 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 

 cisternae. 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. There is free communication 

 between the cerebral and spinal portions of the fluid, as is evidenced by 

 the fact that blood clots appear in the spinal fluid (collected by spinal 

 puncture) when there is hemorrhage into the ventricles of the brain. 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 c.c. in man) 

 as to be apparently of little value in bringing about an alteration in brain 

 volume. Although normally so scanty, its secretion can become re- 

 markably stimulated under certain conditions as in fractures of the base 

 of the skull. Under these conditions in man, it may drain away at 

 the rate of about J3QO c.c. a day or more. In cerebrospinal rhinorrhea as 

 much as 700 c.c. of cerebrospinal fluid may run away in a day. 



The fluid is apparently produced in the choroid plexus, for when the 

 pathways by which the ventricles communicate with the subarachnoid 

 space are obstructed it collects in the ventricles, producing internal hy- 

 drocephalus. Under certain conditions absorption is also very rapid, as 

 shown experimentally by the rate with which physiological saline is 

 absorbed when it is injected into the subarachnoid space. This absorp- 

 tion is believed to occur through various pathways, the most important 

 of which is, directly into the blood stream, through the capillaries lining 



