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118 AyATOMY.

The Skull and its Articulations at Different Periods op
Life.
About the second month of embryonal life the brain-case is divided
into two almost equal compartments by the tentorium cerebelli, which
at this period extends almost perpendicularly from its anterior attach-
ment within the skull. This division shows the posterior or cerebellar
portion of the encephalon at this period to be larger in proportion to
the anterior or cerebral than in the adult. Shortly after the second
month the rapid growth of the parietal bones causes the occipital por-
tion of the cranium to be pushed backward. By the final enlargement
of the frontal bones the anterior or cerebral fossae are completed.
At birth the parietal bones are large in proportion to the other bones
of the head, and their centres of ossification are extremely prominent.
The frontal eminences and the occipital protuberance are also noticeably
convex.
During the first year, to accommodate the enlarging brain, the dome
of the case grows with greater rapidity than the base, the upper portion
of the frontal bone developing to a greater extent than the orbital ])or-
tion, which causes the prominence of the forehead peculiar to children
at this age. At this period the facial bones occupy but about one-eighth
of the entire skull, while at adult life they form almost one-half The
external auditory meatus and the alveolar processes are but partially
developed, and only the anterior deciduous teeth are erupted. The
sutures are more or less open, while the dilferent parts of the bones
formed by separate centres of ossification in many instances are not
united.
During the first year the sutures of the cranial vault are generally so
widely open that the border of one bone can be made to overlap that
adjoining without damage to either or to the brain of the child. Such
overlapping takes place during the birth of a child, and the head may
be sul)jected to considerable compression of various kinds during early
life with comparatively little or no injury.
The sutures according to their location, disappear at diiFerent periods
;
the general ossific development of tlie individual likewise seems to influ-
ence their disappearance. Traces late as the fiftieth or sixtieth year. When a suture is obliterated by
ossification such complete union is called synostosis.
Occasionally some pathological condition will cause a suture to close
prematurely. When tliis occurs the cranium will bulge on the opposite
side, in order to accommodate the brain as it develops.
Wormian Jionc.s (ossa tri and sizes are found within tlie sutures uniting the membranous bones of
the cranial vault. They are rarely found in the face. Their form is
irregular, and their borders are adapted to the suture within which they
are situated. Generally they are small, l)ut occasionally they exceed an
inch in diameter. They are most frccjuently found in the occipito-
parietal suture, where tliey are occasionally met with in considerable
numbers. Their function between membranous bones is similar to
that of cartilage between cartilaginous bones. They have their own