Page:The New International Encyclopædia 1st ed. v. 18.djvu/267

* SKULL. 221 SKULL. 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 mouth, 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 niiiiinuim as compared with the cranial cavity, chielly in consequence 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 bones, viz. the frontal, ethmoid, lachrymal, sphenoid, superior maxillary, malar, and palate, the last contribut- ing very slightly to the huBian orbit, while it is an important constituent in the oi'bit of many animals. For 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 tijpieal 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 piiramidal, and the oval 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- nathous or prognathic, the opposite condition being termed orthognathous or orthognathic. 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. For the skull as a basis of classification in anthropologv', see Anthropometry. The Morphologt of the SKUi-r. is the highest and most difficult problem of comparative anat- omy. Huxley destroyed the archetypal theory, previously held by Owen and others, and estab- lished the newer theorv on sure grounds of actual observation. Taking first the unsegmented cra- nium of a skate or dogfish, with its appended jaws and arches, we find that in development, though the notochord extends into the region of the head, the vertebrse stop short of it; but that on each side of the cranium there arise a pair of cartilaginous bars, the trabeculw or rafters of the future skull; three pairs of cartilaginous cap- sules, nasal, ocular, and auditory, form round the developing sense organs; the na.sal cap- sules unite with the ends of the trabeculie, which are meanwhile uniting below, and growing up at the sides to form the brain-case. The auditory capsules become united with the trabeculse by the appearance of two new masses of cartilage, the parachordals. The first pair of a series of seven or more arches develops an ascending pro- cess, becoming the palato-pterygoid arch or upper jaw. The second pair of arches, the hyoid, is modified to support the jaws, while the rest are modified to support the gills. In the bony skulls of higher vertebrates the chondro-cranium and subjacent arches develop in the same way. The bones originate in two distinct ways: either by actual ossifications or by the ossification of over- lying dermis, known as cartilage bones and mem- brane bones respectively, the latter corresponding to the dermal bones and teeth of ganoid and elas- mobranch fishes. In mammals the ends of the mandibular and hyoid arches lose their suspen- sory function, are taken into the interior of the ear capsule, and are metamorphosed into the auditory ossicles. See Skeleton. Fracture of the Skull may take place either in the vault or at the base of the skull. In frac- ture of the vault the fracture is usually direct, the bone giving way at the point at which it was struck, and the restilt being either a simple fis- sure or a breaking of the bone into several frag- ments (a comminuted fracture). Although frac- tures may be limited to the outer or to the inner table of the skull, they most commonly extend through the whole thickness, and the broken bone is generally driven inward ; and the most ordi- narv form of fracture with depression is that in which several fragments of a somewhat triangu- lar form have their points driven down and wedged into each other, while their bases remain on a level with the surrounding bone. There are no signs by which we can in all cases recognize the existence of fracture of the vault. Fissures in- volving the whole thickness of the vault of the skull occasionally exist without ever having been suspected during life, and even an extensive and comminuted fracture, with great depression of the fragments, may escape notice when hidden under the temporal muscle or under a large ex- travasation of blood. A'hen, however, the frac- ture is accompanied by a wound leading down to the bone, it ma.v. in general, be easily detected. With regard to treatment, it is now an estab- lished rule that simple fractures of the skull with depression and without symptoms are to be let alone. The depression may be so marked as to be easily detected ; and yet so long as there are no symptoms all operative interference, of what- soever form, is carefully to be avoided. If, how- ever, there be a wound leading down to the bone in a depressed fracture without s,-mptoms, im- mediate operative interference is called for. When a depressed fracture is accompanied by primary brain-symptoms an operation for the purpose of raising or removing the depressed fragments is usually necessary. If, however, the