668 



THE NEKVOUS SYSTEM. 



mater. Outside the cranium these prolongations of the membrane blend with the 

 fibrous sheaths of the nerves, and likewise become connected with the periosteum on 

 the exterior of the skull. In the child, during the growth of the cranial bones, and 

 also in old age, the dura mater is more adherent to the cranial wall than during the 

 intermediate portion of life. 



The cranial dura mater is composed of two layers intimately connected with 

 each other, but yet capable of being demonstrated in most regions of the cranium. 

 Along certain lines these two layers separate from each other so as to form channels 

 lined with endothelium. These channels are the venous blood-sinuses which receive 

 the blood from veins which come from various parts of the brain. They are described 

 in the section dealing with the Vascular System. 



Strong fibrous partitions or septa are given off along certain lines from the deep 



Internal carotid artery 

 Basilar venous plexus 



Inferior petrosal sinus | 



Superior petrosal sinus 



Sigmoid part of 

 transverse sinus 



FIG. 595. SAGITTAL SECTION THROUGH THE SKULL, A LITTLE TO THE LEFT OF THE MEDIAN PLANE 

 to show the arrangement of the chira mater. 



The cerebral nerves are indicated by numerals. 



surface of the dura mater. These project into the cranial cavity, and subdivide 

 it partially into compartments which all freely communicate with each other, 

 and each of which contains a definite subdivision of the brain. These septa are : (1) 

 the falx cerebri ; (2) the tentorium cerebelli ; (3) the falx cerebelli ; and (4) the 

 diaphragma sellae. 



The falx cerebri is a sickle-shaped partition which descends in the great longi- 

 tudinal fissure between the two hemispheres of the cerebrum. In front it is narrow, 

 and attached to the crista galli of the ethmoid bone. As it is followed backwards 

 it increases in breadth, and posteriorly it is attached, along the median plane, to the 

 upper surface of the tentorium. The anterior narrow part of the falx is frequently 

 cribriform, and is sometimes perforated by apertures to such an extent that it almost 

 resembles lace-work. Along each border it splits into two layers, so as to enclose a 

 blood-sinus. Along its superior convex attached border runs the superior sagittal 

 sinus ; along its concave free border sometimes courses the much smaller inferior 



