PECULIARITIES OF BLOOD SUPPLY IN CERTAIN VISCERA 249 



Although normally so scanty, its secretion can become remarkably stim- 

 ulated 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 

 200 c.c. a day or more. 



The fluid is apparently secreted from the choroid plexus, for when 

 the pathways by which the ventricles communicate with the subarach- 

 noid space are obstructed, it collects in the ventricles, producing internal 

 hydrocephalus. Under certain conditions its absorption is also very 

 rapid, as shown experimentally by the rapidity with which physiologic 

 saline is absorbed when it is injected into the subarachnoid space. This 

 absorption is believed to occur through the Pacchionian bodies, which 

 are minute sac-like protrusions of the arachnoid into the interior of a 

 venous sinus. The membrane that separates blood and cerebrospinal 

 fluid is extremely thin at these places (Fig. 79). 



Physical Conditions of Circulation 



On account of these anatomic peculiarities, the physical factors con- 

 trolling the circulation of blood to the brain are considerably different 

 from those obtaining in any other part of the body, with the possible 

 exception of the bones. In other vascular areas, we have seen that, when 

 dilatation or constriction of the vessels occurs, a marked increase or 

 diminution of the volume of the part becomes evident. Such a change 

 in volume is evidently impossible in the case of the brain because of 

 the rigid cranium in which it is contained. In fact, from a physical 

 point of view we must consider the blood vessels of the brain as pro- 

 jecting into a rigid case filled with incompressible material. Under 

 these conditions it is obvious that the vessels as a whole could neither 

 contract nor dilate without some increase or decrease in the volume of 

 the contents of the cranial cavity (Leonard Hill 19 ). 



Some have thought that the cerebrospinal fluid as it flows into or out 

 of the spinal cord might accomplish this alteration in the cranial con- 

 tents, but the relatively small amount of available cerebrospinal fluid, 

 the smallness of the openings between the brain and the spinal cord, and 

 the lack of experimental evidence that such changes in Volume of cere- 

 brospinal fluid in the spinal cord do actually occur, all stand in contra- 

 diction to such a view. However, although the vessels as a whole might 

 not contract or expand, yet some vessels, like the arteries, might con- 

 tract simultaneously with a corresponding dilatation of other vessels, 

 such as the smaller cerebral veins. In admitting the possibility of some 

 reciprocal relationship between arteries and veins, we must remember 

 that it is only before the well-protected sinuses are reached that a 

 change in the caliber of the veins would be possible. But it is difficult 



