OSSIFICATION), ap proach one another by gradual enlarge ment and become united in various ways, so as to form a continuous and ultimately an un yielding bony case admirably adapted for the defense of the brain, for the accom modation of the organs of special sense, and for the attachment of the ligaments and muscles by which the skull is supported and moved on the spine. At the period of birth most of the principal bones have grown into apposition with their neighbors, forming the sutures, but one large vacuity remains at the meeting-point of the parietal and frontal bones, which is termed the anterior fontanel. which does not close till the second year after birth, and sometimes re mains open much longer. There are two fon tanels in the mesial line, as shown in Fig. 2, and two lateral fontanels on each side.
After the sutures have been formed and the skull has acquired a certain thickness a process of resorption commences in the interior of the bones, and reduces the originally dense structure to a more or less cellular or cancellated state. The interior thus altered is called the dipioe. and by this change the weight of the skull is much diminished, while its strength is scarcely affected.
The growth of the skull after the seventh year proceeds slowly, lint a slight increase goes on to about the age of twenty. The skull bones are freely supplied with blood from arteries, which pass from the dura mater internally and the pericranitnn externally, through numerous fora mina. the blood being returned by veins which take various directions.
The base of the skull, whether seen from within or from below. presents many objects of physio logical interest in relation to the nervous system. As seen from within the base presents on each side three fosse, corresponding to the anterior and middle lobes of the cerebrum and to the cerebellum. These fossce are marked, as is the whole skull-cap. by the cerebral convolutions, and they contain numerous foramina and fissures which give passage to nerves and blood vessels. The external or outer surface of the base of the skull, if we consider it from before backward, is formed by the palate processes of the superior maxillary and palate bones; the comer; the pterygoid, and spinous processes of the sphenoid and part of its body; the under surface of the temporal bones; and the occipital bone. The hard palate is formed by the palate processes of the superior maxillary bone.
The anterior region of the skull, which forms the face, is of an irregularly oval form, and the bones are so arranged as to inclose the cavities for the eyes, the nose, and the moutb, and to give strength to the apparatus for masticating the food. The size of the face and the capacity
of the cranial cavity stand in an inverse ratio one to another, as may be readily seen by com paring vertical sections (through the mesial line) of human and other mammalian skulls; and if, in place of mammalian skulls, we take skulls of lower vertebrates (the crocodile, for example), this ratio is far more striking. In man the face is at its minimum as compared with the cranial cavity, chiefly in consequenee of the facial bones being arranged in a nearly vertical manner beneath the cranium, instead of project ing in front of it. The human face is also re markable for its relatively great breadth, which allows the orbits for the reception of the eyes to be placed in front instead of on the sides of the head, and renders their inner walls nearly par allel, thus contributing, through the parallelism of the optic axes, to that clear, accurate, and steady vision which results from the ready con vergence of the eyes upon every object. Each orbit is of a pyramidal form, with the apex be hind, and is composed of seven hones, viz. the frontal, ethmoid, lachrymal, sphenoid, superior maxillary, malar, and palate, the last contribut ing very slightly to the hunian orbit, while it is an important constituent in the orbit of many animals. Fo• description of the nasal cavities, see NOSE.
The different varieties of mankind present cer tain well-marked and characteristic peculiarities in the form of the skull. There are three typical forms of the skull which seem to be well estab lished from the examination and comparison of a large number of crania: the prognathous, the pyramidal, and the ova/ or elliptical cranium. When the upper jaw slopes forward the insertion of the teeth, instead of being perpendicular, is oblique. A skull with this peculiarity is prog pathous or prognathic, the opposite condition being termed o•thognathous or o•tIDqnothic. The negro of the Guinea coast and the negrito of Australia present the prognathous character in its most marked form. The pyramidal form is characterized by the breadth and flatness of the face. which, with the narrowness of the forehead, gives this shape to the head. The Mongolian and Eskimo skulls belong to this type. The oval or elliptical type is that which is presented by the natives of Western or Southern Europe, and which is not distinguished by any particular feature so much as by the absence of the longi tudinal projection of the first type, or the lateral projection of the second, and by a general sym metry of the whole configuration. Fo• the skull as a basis of classification in anthropologu•, see ANTHROPONIETRY.