SPECIAL ANATOMY. 153 



the superior tlie nerve emerges, througli the inferior the 

 Eustachian tube communicates with the middle ear. It is 

 just at the root of the styloid process of the petrous tem- 

 poral bone. The inner surface of the os triquatrum 

 presents an eminence (ossific tentorium) consisting of the 

 convergence of three irregular bony ridges from which 

 prominences run. On either side of the wall of the cavity 

 one runs to the floor of the cranium, and one runs forwards 

 along the central line to the crista galli process. In other 

 parts the roof and sides of the cranium present slight pro- 

 minences which separate broad shallow depressions (digital 

 imjpressions) which are adapted to the convolutions of the 

 brain. 



The brain is covered by three membranes — dura mater, 

 arachnoid, and pia mater. The dura mater is externally 

 placed, and is a dense fibrous membrane, which forms the 

 internal periosteum of the skull. It presents processes, 

 some of which run outwards, others inwards. Those pass- 

 ing outwards establish its connection with the periosteum 

 of the outer surface of the bones by passing through the 

 immovable joints between them ; this accounts for the 

 difficulty experienced in separation of the dura mater from 

 the bones at the sutures. The processes running inwards 

 are two in number. Falx cerebri is a sickle-shaped fold, 

 running from the superior extremity of the crista galli, 

 and increasing in size until it terminates posteriorly in 

 becoming attached to the ossific tentorium. It is broadest 

 above and grows narrow below, forming a sharp edge. It 

 fits into the superior longitudinal fissure of the brain. 

 Along its attached margin runs the superior longitudinal 

 sinus, along its free margin the inferior longitudinal sinus. 

 These are connected together posteriorly by the straight 

 sinus, into the anterior part of which vena Galeni empties 

 itself, after running from the velum interpositum and from 

 the upper surface of the corpus callosum. These sinuses are 

 dilated veins situated in the dura mater, which splits to 

 accommodate them. They are lined by epithelium con- 

 tinuous with that of the other veins, and at various parts 

 we see fibrous cords (cordce Willissii), which run across 

 them to prevent undue distension. From the lateral parts 

 of the ossific tentorium run folds to the floor of the 

 cranium, the tentorial membrane. They are attached to 

 the inner ridges of the parietal bones, and project into the 



