DURA MATER. PROCESSES. 407 



proportion to their size, producing various effects upon contiguous parts. 

 For example, when small they remain free or constitute a bond of ad- 

 hesion between the visceral and parietal layer of the arachnoid ; when 

 of larger size they produce absorption of the dura mater, and as the 

 degree of absorption is greater or less, they protrude through that 

 membrane, and form depressions on the inner surface of the cranium, 

 or simply render the dura mater thin and cribriform. Sometimes they 

 cause absorption of the wall of the longitudinal sinus, and projecting 

 into its cavity give rise to the granulations described in relation to that 

 channel. 



If the student cut through one side of the dura mater, in the di- 

 rection of his incision through the skull, and turn it upwards towards 

 the middle line, he will observe the smooth internal surface of this 

 membrane. He will perceive also the large veins of the hemispheres 

 filled with dark blood, and passing from behind forwards to open into 

 the superior longitudinal sinus ; and the firm connection, by means of 

 these veins and the Pacchionian bodies, between the opposed surfaces 

 of the arachnoid membrane. If he separate these adhesions with his 

 scalpel, he will see a vertical layer of dura mater descending between 

 the hemispheres; and if he draw one side of the brain a little out- 

 wards, he will be enabled to perceive the extent of the process of mem- 

 brane, which is called the falx cerebri. 



The processes of dura mater which are sent inwards towards the 

 interior of the skull, are the falx cerebri, tentorium cerebelli, and 

 falx cerelelli. 



The Falx cerebri (falx, a sickle), so named from its sickle-like ap- 

 pearance, narrow in front, broad behind, and forming a sharp curved 

 edge below, is attached in front to the crista galli process of the 

 ethmoid bone, and behind to the tentorium cerebelli. 



The Tentorium cerebelli ( tentorium, a tent) is a roof of dura mater, 

 thrown across the cerebellum and attached at each side to the margin 

 of the petrous portion of the temporal bone ; behind to the transverse 

 ridge of the occipital bone, which lodges the lateral sinuses ; and to the 

 clinoid processes in front. It supports the posterior lobes of the 

 cerebrum and prevents their pressure* on the cerebellum, leaving only 

 a small opening anteriorly, for the transmission of the crura cerebri. 



The Falx cerebelli is a small process, generally double, attached to 

 the vertical ridge of the occipital bone beneath the lateral sinus, and to 

 the tentorium. It is received into the indentation between the two 

 hemispheres of the cerebellum. 



The layers of the dura mater separate in several situations, so as to 

 form irregular channels which receive the venous blood. These are the 

 sinuses of the dura mater, which have been described at page 370. 



The student cannot see the tentorium and falx cerebelli until the 

 brain is removed ; but he should consider the attachments of the for- 



* In leaping animals, as the feline and canine genera, the tentorium forms a 

 bony tent. 



