762 THE NERVOUS SYSTEM 
which enclose the dorsal longitudinal sinus. Its ventral border is concave and 
lies over the corpus callosum. The falx is thick above, but much thinner below, 
and is in some places cribriform.! 
The tentorium cerebelli is a crescentic transverse fold which occupies the 
transverse fissure between the cerebellum and the cerebral hemispheres. It is 
attached dorsally to the tentorium osseum and laterally to the petrosal crest. Its 
ventral border is thin, concave, and free; it forms the dorsal and lateral boundaries 
of an opening (Incisura tentoril) in which the mid-brain is situated. 
The diaphragm sellz is a thickening of the dura which roofs over the pitui- 
tary fossa; it covers the pituitary body and the cavernous and intercavernous 
sinuses. It is perforated centrally by an opening (Foramen diaphragmatis) for 
the infundibulum. 
The falx cerebelli, a sickle-shaped fold which projects into the median notch between the 
cerebellar hemispheres in man, is not present in the domesticated animals. There is instead 
merely a sight thickening of the dura. 
The spinal dura mater (Dura mater spinalis) forms a tube around the spinal 
cord from the foramen magnum to the second or third segment of the sacrum. It 
is separated from the periosteum of the spimal canal by a considerable epidural 
space (Cavum epidurale), which is occupied by fatty connective tissue and veins. 
It is held in position chiefly by the sheaths which it furnishes to the roots of the 
spinal nerves, and in its anterior part by two ligaments; the latter connect it with 
the ventral atlanto-occipital membrane and with the dens of the axis. It is large 
in proportion to its contents, but its diameter is not uniform. It is largest in the 
atlas, small in the thoracic region, and becomes very small in its terminal part, 
where it encloses the delicate filam terminale of the spinal cord. 
The subdural space (Cavum subdurale) is the cavity between the inner surface 
of the dura mater and the arachnoidea. It is a mere capillary space which contains 
just sufficient fluid to moisten its surfaces; this fluid is usually regarded as lymph, 
which is replenished by filtration through the walls of the blood-vessels. The 
space is in communication with the lymph paths of the nerve sheaths. 
THE ARACHNOIDEA 
The arachnoidea is a very delicate and transparent membrane which is situated 
between the dura and pia mater. Its outer surface forms the inner wall of the sub- 
dural space and is covered by a layer of endothelium continuous with that of the 
opposed surface of the dura mater. Between it and the pia mater is the subarach- 
noid space (Cavum subarachnoicdeale), which contains the cerebrospinal fluid. An 
inner surface can scarcely be said to exist, since deeply the membrane becomes a 
reticulum of fine fibers which traverse the subarachnoid space and are attached 
to the pia mater. The arachnoidea furnishes sheaths to the cerebral nerves from 
their superficial origins to a variable but usually short distance beyond the emer- 
gence from the sac. In the case of the optic nerve this sheath extends to the eyeball. 
The cerebral arachnoidea (Arachnoidea encephali), except in the case of the 
great longitudinal and transverse fissures, does not dip into the sulci on the surface 
of the brain. On the summits of the gyri it is so closely attached to the pia mater 
that the two form practically a single membrane. Its outer part bridges over the 
sulci, and here the subarachnoid space is partially divided up by the loose arach- 
noid tissue into intercommunicating cavities. In certain situations the arach- 
noidea is separated from the pia by spaces of considerable depth and extent. These 
enlargements of the subarachnoid space are termed subarachnoid cisterns (Cisterne 
subarachnoidales). In them the subarachnoid tissue does not form a close network, 
1 In the horse the posterior part of the falx cerebri does not extend to the corpus callosum, and 
the cerebral hemispheres are here in contact and adherent to each other over a small area. In the 
other animals the falx does not descend so far as in the horse. 
