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120 ANATOMY.
in by Wormian bones ; usually all traces of the fontanelles disappear
about the fourth year.

The Walls of the Brain-case.^
The bones forming the walls of the brain-case are composed of two
plates of compact tissue, an outer and an inner, with intervening can-
cellated tissue, called diploe, between them.
The Older or Fibrous Plate or Table is thick and tough, and rough-
ened in different places for the origin and insertion of muscles. It is
also covered by minute orifices for the attachment of the pericranium
(periosteum) and entrance of the nutrient vessels.
The Inner Plate or Vitreom Table is thinner, smoother, closer-grained,
and more brittle than the outer, and has a glossy appearance. The
minute orifices are not so numerous as they are externally, and they
give attachment to the dura mater, which acts as the internal peri-
osteum.
T»
tissue situated
The Diploe (see Fio-. 115, veins) is the cancellated 11 I'l
between the external and the internal plates. It gives the bone light-
ness, and at the same time acts as a cushion to diffuse, and thus mod-
erate, shocks. It is extremely vascular, and gives passage to numerous
blood-vessels, which communicate with both the pericranium and the
dura mater in such manner that death of the pericranium is not always
followed by death of the bone. It is unevenly distributed throughout
the different parts of the skull, being thick in some places, as in the
region of the greater portion of the occipital bone and the mastoid por-
tion of the temjioral bone, while it is entirely absent in others, as in
portions of the orbital plates of the frontal bone and the glenoid fossae
of the temporal bones.
The Internal Surface of the Brain-cam is smooth, glossy, and marked
by digitate depressions corresponding to the convolutions of the brain.
The interior of the skull is separated into two principal divisions—first,
the roof or dome ; and second, the floor or base.
The Dome or Vertex of the brain-case is oval-shaped and vaulted,
generally wider behind than in front, and made up of the frontal, two
parietal, and a jiortion of tlie occipital bones. The sutures between
these internally belong to the variety known as harmonia. It is
marked by the superior longitudinal groove, which extends from its
anterior to its posterior portion. This groove is deeper in front and
behind than in its central ]iortion. The surfaces of the bones are fur-
rowed for the accommodation of the meningeal vessels, and marked by
depressions of different depths for the lodgment of the Pacchionian
bodies.
The Floor or Base of the internal portion of the brain-case is divided
into three pairs of fossa;, tlie anterior, middle, and ])osterior (Fig. 63).
The Anterior Fossa', arc formed by the cribriform plate of the ethmoid,
the orbital plates of the frontal, the lesser wings, and a portion of the
* For detailed particulars of the processes, surfaces, and foramina of the bones form-
ing the brain-case see description of individual hones. Foramina formed by the union
of two or more bones will be described under this heading.