Literary Notices. cxxxix 



Subdural Membranes and Intercranial Pressure.^ 



Cerebral atrophy is of common occurrence with membrane for- 

 mation, and the loss of support thus caused is given by many as the 

 occasion of the engorgement and rupture of the vessels. The com- 

 pensating hypertrophy of the skull in chronic insanity is another re- 

 sult. In cases of cerebral atrophy the cerebrospinal fluid tends to 

 increase in a compensating manner. 



On account of being contained in rigid walls the bra'n needs 

 special arrangements to permit of shrinkage and expansion. The 

 brain is essentially erectile tissue and the adjustment is affected by the 

 cerebro-spinal fluid which exists normally in the meshes of the pia- 

 arachnoid and the ventricles and does not exceed four ounces in the 

 healthy adult. The source of the fluid is the plexuses. From the 

 fourth ventricle "it passes by the foramen of Magendie to the inferior 

 cerebellar lake, and round the cerebellum and crura cerebri to the 

 superior cerebellar lake. This communicates along the peripeduncu- 

 lar and basilar canals with the great central lake at the base of the 

 brain. From the sides of the central lakes there arise the two Sylvian 

 lakes, which are extended into the Sylvian and Rolandic rivers. 

 From these rivers tributaries extend along the minor sulci and by 

 these means, though somewhat circuitous, cerebro-spinal fluid is car- 

 ried to every part of the surface of the brain." The fluid is absorbed 

 by the Pacchionian glands and is secreted into the veins and sinuses. 

 The rate of absorbtion is more rapid than that of secretion and seems 

 to depend on the pressure ot the fluid. 



The blood and not this fluid plays the active part in regulating 

 intercranial pressure. 



It is found that the size of a blood vessel varies greatly within a 

 brief time in paralytics. Now with atrophied brain and increased 

 fluid it would appear reasonable that a sudden engorgement followed 

 by constriction of the vessels might produce a demand for increase of 

 cerebro-spinal fluid greater than could be met and the vessels of the 

 dura would become gorged and ruptured. The seat of " ialse-mem- 

 branes " is limited to the parietal bone; this the author explains as due 

 to the habitual position which leads the brain to tend to fall away 

 from the vault by gravitation, enhancing the tendency to tear in this 

 region. 



'Robertson, G. M. The Formation of Subdural Membranes or Pachy- 

 meningitis hemorrhagica. Journ. Metital Science, XXXIX. N. S. 130, July. 

 1893. 



