THE MENINGES. 



21 



dura to the skull. This attachment is firmest on the base of the skull. On the vault, 

 after an opening has been made through the skull by a trephine, the dura can be 

 readily separated from the bone by means of a thin, flat, steel spatula. On account of 

 the small size of the vessels passing from the dura to the bone, this procedure is not 

 usually accompanied by much hemorrhage. In separating the dura from the base of 

 the skull, as is done in operations on the Gasserian ganglion, the bleeding from this 

 source is often quite free. The dura is liable to be torn in lifting it from the bone 

 if the greatest care is not exercised. 



The middle meningeal artery, at a distance of 4 cm. ( i ^ in. ) posterior to the 

 angular process of the frontal bone and about the same above the zygoma, usually 

 passes within the bone for a distance of i or 2 cm. Therefore, in operating in the 

 temporal region, if the dura is detached the vessel will be torn and free bleeding will 



FIG. 27. Vault of the skull opened and brain removed, showing the falx cerebri and tentorium. 



follow. The vessel is liable to be torn in endeavoring to remove bony fragments in 

 fractures of this region. The dura is also more firmly attached in the median line; 

 and on each side of the median line are the depressions in the parietal bone which 

 lodge the Pacchionian bodies. The largest are usually located at a distance of from 

 2 to 5 cm. posterior to a line drawn across the skull from one external auditory 

 meatus to the other. They are prolongations from the arachnoid and are surrounded 

 by blood from the longitudinal sinus. 



The Arachnoid also called Arachnopia or Parietal Layer of the Pia. 

 The arachnoid is a thin fibrous membrane, which passes over the convolutions of 

 the brain and does not dip into the sulci between. It is more marked on the base 

 than on the convexity of the brain. It is not attached to the dura above, and this 

 subdural space, while moist, contains little or no free fluid. Hemorrhages do not 

 occur into this space unless the membranes are torn, because the bleeding from the 

 vessels of the dura is always epidural and the arachnoid derives its nourishment from 

 the pia mater below, so that hemorrhages start beneath the arachnoid, but may rupture 

 through the arachnoid into the subdural space. From its under surface, fibrils of loose 



