340 NERVOUS SYSTEM. 



wards the centre of the cavity of the cranium. They are as 

 follow: 



The Falx Cerebri separates the hemispheres of the brain, and 

 consequently, precisely under the middle line of the head. 

 Its shape is well indicated by its name. It commences by a 

 small point from the middle of the body of the sphenoid bone, 

 and continues to arise alojig the crista galli, the spine, and 

 middle line of the frontal bone, the sagittal suture and the up- 

 per limb of the occipital cross, till it reaches the internal occi- 

 pital protuberance. It is about an inch broad in front, where 

 it begins, but it increases continually, though gradually, in 

 breadth till its termination, where it is two or two and a half 

 inches wide. It is strongly fastened along the crista galli, and 

 at the foramen ccecum, and being also fastened behind to the 

 tentorium, (with which it is continuous,) as well as along the 

 intermediate points of bone, it is kept in a state of strict tension, 

 which does not admit of its wavering to one side or to the other. 

 Its inferior margin is very concave, and goes to within a small 

 distance of the corpus callosum. There are sometimes consi- 

 derable apertures in it, through which the flat surfaces of the 

 hemispheres come in contact. 



The Tentorium Cerebelli, another process of the internal la- 

 min* of the dura mater, is placed transversely across the pos- 

 terior part of the cranium, and separates the cerebellum from 

 the posterior lobes of the cerebrum. It is continuous with the 

 posterior end of the falx major, whereby these two processes ex- 

 ercise a mutual tension. The tentorium is, therefore, kept convex 

 above and concave below. 



Its form is crescentic ; its outer circumference is extended 

 along the horizontal limbs of the occipital cross, and along the 

 superior corner or margin of the petrous bones to the posterior 

 clinoid process. The internal circumference is much smaller 

 and unattached, and being placed immediately behind the sella 

 turcica, it leaves an opening (the Foramen Ovale) which is near- 

 ly of the same size with, and occupied by the tuber annulare 

 and the crura cerebri. The anterior extremities of the crescent 

 are continued from the posterior clinoid process to the anterior 



