THE MEMBRANES OF THE BRAIN. 1199 



surrounding the optic nerve through its entire length is noteworthy on account 

 of its unusual thickness and completeness (page 1223). 



The two layers of which the dura is composed are, for the most part, so closely 

 united that only a single membrane is demonstrable. The division into two layers, 

 however, is evident in certain localities, particularly in the middle fossa at the base of 

 the skull. Here, on each side of the body of the sphenoid bone, the layers separate 

 to form the cavernous sinus and, within the sella turcica, enclose the pituitary body. 

 Over the apex of the petrous portion of the temporal bone they include between 

 them a space, the cavum Meckelii, which lodges the Gasserian ganglion, whilst over 

 the aqueductus vestibuli the dilated end of the endolymphatic duct, the saccus 

 endolymphaticus, continued from the membranous labyrinth, lies between the two 

 layers of the dura. Further, along the lines of its attachment to the skull beneath 

 the sagittal suture, to the crucial ridges on the occipital bone and to the ridges of 

 the petrous bones, the inner layer of the dura separates from the outer and forms 

 partitions, which project inward and imperfectly subdivide the cranial cavity into 

 compartments occupied by the larger divisions of the brain, as well as enclose the 

 blood-spaces, known as the dural sinuses. These spaces have been described with 

 the veins (page 867) and will be here only incidentally mentioned in connection with 

 the partitions in which they lie. On either side of the superior longitudinal sinus, the 

 layers of the dura exhibit local areas of separation, which prolong laterally the lumen 

 of the venous channel. These parasinoidal spaces, the lanaice venos(S laterales, are 

 of consequence as receiving many of the cerebral veins and as affording additional 

 localities in which the Pacchionian bodies may come into relation with the blood-* 

 stream. The septa thus formed by duplicatures of the inner dural layer are : ( i ) the 

 falx cerebri, (2) the tentorium cerebelli, (3) \h.Q falx cerebelli, and (4) the dia- 

 phragma sellce. 



The falx cerebri is a sickle-shaped partition which occupies the greater part of 

 the longitudinal fissure separating the cerebral hemispheres. Its upper and longer 

 border is attached in the mid-line and extends from the cristi galli of the ethmoid 

 bone in front to the internal occipital protuberance behind and encloses the sjiperior 

 longitudinal sinus. The latter channel appears triangular in cross-section (Fig. 

 1034), the upward placed base being the outer or parietal layer of the dura and the 

 sides the separated lamellae of the falx. The lower and shorter border of the falx is 

 free and more sharply arched than is the upper, and extends from the hind part of 

 the cristi galli to the highest point of the tentorium. Within its posterior half it 

 encloses the inferior longitudinal sinus. The base of the falx is oblique, approxi- 

 mately at 45° with the horizontal plane, and attached to the upper surface of the 

 tentorium in the sagittal plane. Along this junction lies the straight si7uis. The 

 narrow forepart of the falx is the thinnest portion of the partition and is often, more 

 especially during the latter half of life, the seat of perforations, which may be so 

 numerous as to reduce this part of the septum to a fenestrated membrane. Occasional 

 deposits of true bone are found within the falx, which may be without pathological 

 significance and represent the constant ossification of this partition seen in some 

 aquatic mammals. 



The tentorium cerebelli is the large tent-like partition that roofs in the pos- 

 terior fossa of the skull and separates the cerebellum from the overlying posterior 

 parts of the cerebral hemispheres. In its general form it is crescentic, the longer 

 convex border lyings behind and attached to the posterior and lateral margins of the 

 posterior cranial fossa, and the shorter concave anterior border curving backward 

 and upward from the anterior clinoid processes. The upper surface of the tentorium 

 is attached by its entire width to the falx cerebri along the mesial plane, and in this 

 manner the partition is maintained in a tensed condition. The sides of the tent-like 

 fold are, however, not simply flat, but present a slight downwardly directed convexity 

 in both the sagittal and frontal planes. The peculiar curvature of the under surface of 

 the tentorium is reproduced, in reversed relief, by the upper aspect of the cerebellum' 

 which is accurately applied to the partition. 



The posterior border of the tentorium is attached to the horizontal ridge crossing 

 the occipital bone ; farther outward, on each side, it is fixed to the postero-inferior 

 angle of the parietal bone and, continuing forward and inward, to the upper border 



