256 THE CIRCULATION OF THE BLOOD 



the subaraclmoid spaces of the brain and cord. There is no evidence to 

 support the view that absorption occurs through the Pacchionian bodies. 

 A smaller degree of absorption occurs by way of the lymphatics which 

 run between the interior of the cranial cavity and the deep cervical 

 glands, some also occurs into the lymphatics of the upper nasal passages 

 and the perineural sheaths. 



The cerebrospinal fluid 1 is believed to undergo a slow circulation from 

 the ventricles through the foramen of Magendie into the subarachnoid 

 spaces of the spinal cord down which it travels on the posterior aspect, 

 and then ascends on the anterior aspect where the greater part of its ab- 

 sorption occurs. The biochemical properties of this important fluid will 

 be found elsewhere (page 121). 



Physical Conditions of the Intracranial Circulation 



Considered from a physical standpoint the circulation through the 

 brain has been recognized for long to be unique in comparison with that 

 of any other organ or tissue in the body with the exception of the bone 

 marrow. Encased in the rigid cranium, the volume of the brain can not, 

 like that of other vascular areas, expand and contract in proportion to 

 changes in the blood supply ; neither can the caliber of its blood vessels 

 become altered, unless some special mechanism may exist whereby a part 

 of the cranial contents are quickly expelled from and aspirated into the 

 rigid case. In a general way, the physical conditions of the intracra- 

 nial circulation are similar to those existing in a flask full of water and 

 having a thin-walled rubber tube suspended in the water with its free 

 ends connected with glass tubes passing through the stopper of the flask. 

 If fluid be made to circulate through the tubing, no change in the caliber 

 can be produced by altering the pressure of inflow; but the rate of dis- 

 charge from the other end of the tube will be proportionate to the pres- 

 sure. Although the tubing itself is readily distensible and elastic, these 

 properties are entirely annulled by the incompressible fluid in which 

 the tube is suspended. 



If any expansion or contraction of the tubing as a whole is to occur, 

 provision must be made for changes in the volume of fluid in the flask 

 by inserting in the stopper a third tube connected with an overflow 

 flask, and in applying this second model to represent the circulatory 

 conditions as they exist in the brain, the question arises as to whether 

 the cerebrospinal fluid which lies in the large subarachnoid spaces at 

 the base of the brain and in the ventricles, by communicating through 

 the foramen of Magendie with the spaces surrounding the spinal cord, 

 may not be capable of functioning as the overflow fluid. This is at 

 least conceivable, especially when one bears in mind that some outflow 



