508 



DISSECTION OF THE HEAD. 



Inner 

 surface 



and struc- 

 ture. 



Processes. 



Falx 

 cerebri : 



form and 

 attach- 

 ments ; 



borders ; 



sinuses in it. 



Superior 

 longitudinal 

 sinus : 



situation 

 and ending ; 



its interior ; 



veins open- 

 ing into it. 



is then to be raised on the right side towards the top of the 

 head ; and the veins connecting it with the brain may be broken 

 through. 



The inner surface of the dura mater is free and smooth, being 

 separated from the arachnoid (the second of the coverings of the 

 brain) by the cavity known as the subdural space, although the two 

 membranes are in the natural condition closely applied to one 

 another. The fibrous tissue of which the dura mater is composed 

 is so arranged as to give rise to two strata, an external (or periosteal] 

 which adheres to the bones, and an internal (or meningeal) which 

 is lined by an epithelium similar to that 011 serous membranes 

 At certain spots these layers are slightly separated, and form 

 thereby the spaces or sinuses for the passage of the venous blood. 

 Moreover, the innermost layer sends processes between different 

 parts of the brain, forming the falx. tentorium, &c. 



The falx cerebri (fig. 187, p. 512) is the median sickle-shaped 

 process of the dura mater, which dips in between the hemispheres 

 of the large brain. Its form and extent will be evident if the right 

 half of the brain is gently separated from it. Narrow in front, 

 where it is attached to the crista galli of the ethmoid bone, it 

 widens behind, and joins a horizontal piece of the dura mater 

 named the tentorium cerebelli. Its upper border is convex, and 

 is fixed to the middle line of the skull as far backwards as the 

 internal occipital protuberance ; and the lower or free border is 

 concave and turned towards the central portion of the brain (corpus 

 callosum), with which it is in contact inferiorly. 



In this fold of the dura mater are contained the following 

 sinuses : the superior longitudinal along the convex border, the 

 inferior longitudinal in the hinder part of the lower edge, and the 

 straight sinus at the line of junction between it and the tentorium 

 (fig. 187). 



The SUPERIOR LONGITUDINAL SINUS (fig. 187, 6) extends from 

 the ethmoid bone in front to the internal occipital protuberance 

 behind. Its position in the convex border of the falx will be 

 made manifest by the escape of blood through numerous small 

 veins, when pressure is made from before backwards with the 

 finger along the median part of the dura mater. 



Dissection. The sinus is now to be opened by cutting into it 

 from above along the middle line and by detaching the dura from 

 the bone down to the internal occipital protuberance behind. 



When the sinus is opened it is seen to l^e narrow in front, and 

 to widen behind, where it ends in a dilatation termed the torcular 

 Herophili on one side (more frequently the right) of the internal 

 occipital protuberance. Its cavity is triangular in form, with the 

 apex of the space turned to the falx ; and across it are stretched 

 small tendinous cords chordce Willisii near the openings of some 

 of the cerebral veins. Frequently small Pacchionian bodies project 

 into the sinus. 



The sinus receives small veins from the substance of the skull 

 and dura mater, and larger ones from the brain ; and the blood 



