Page:The New International Encyclopædia 1st ed. v. 18.djvu/266

SKXJA. 220 SKULL. are moderately large, about 20 inches long and about 4 feet in extent of wings. The plumage is dusky above and usually white below. Con- sult Selous, Bird Watchiitg (London, 1900). SKULL ( Icel. skSl, bowl, cup ; connected with AS. Srcale, Eng. scale, bowl, dish of a balance, and with AS. sceahi. sceale, OHG. scala. Ger. Scliale, husk, scale, Goth, skalja. tile, OChurch Slav. skoUka, mussel, Litli. skelti. to split). The bony framework of the liead. It is divided into two parts, the cranium and the face. In human anat- sule, which soon becomes converted into flakes of bone, which aft'ord protection for the brain, while the intervening portions, which remain membranous, permit the skull to expand as its contents enlarge. The formation of these bony Hakes on the convexity of the cranium is soon followed by the ap- pearance of osse- ous nuclei in the cartilage at the base, corresponding to the future oc- cipital and sphe- noid bones. Lastly, the various bones, some originating in membrane and some in cartilage (see Ossification ) . ap- proach one another by gradual enlarge- ment and become imited in various ^"'- ■'• Anterior fontanel wavs, so as to form ■^' , ^ , • ' rior loutanel: Fig. 1. BIDE VIEW of BTMAN SKfLL. 1, Frontal bone; 2. parietal bone: 3, occipital bone; 4. temporal bone ttsqiiaraoue portion); i", Do. (mastoid por- tion); 5, sphenoid bone: 6, malar bone; 7, nasal bone; 8. euperior maxillary or jaw bone; 9, inferior maxillary or jaw bone. oniy it is customary to describe the former as consisting of 8 and "the latter of 14 bones; the 8 cranial bones, which constitute the brain-case, being the occipifal, two parietal, frontal, two temporal, sphenoid, and ethmoid; while the 14 facial bones are the two ndsal. two superior max- illary, two lachri/mal, tiro malar, two palate, tn-o inferior tnrhtnated. vomer, and inferior miijc- illary. The bones of the ear, the teeth, and the Wormian bones are not included in this enumera- tion. At a very early period of foetal existence the cerebrum is inclosed in a membranous cap- sule external to the dura mater and in close contact with it. This is the first rudiment of the skull, the cerebral portion of which is conse- quently formed before there is an}' indication of a facial part. Soon, however, four or five proc- esses jut from it on eacli side of the mesial line, which grow downward, incline toward each other, and unite to form a series of inverted arches, from which the face is ultimately devel- oped. Imperfect development or ossification of these rudimentary parts of the face gives rise to the peculiarities known as harelip (q.v. ) and cleft-palate, or in extreme cases to the form of monstrosity termed cyclops, in which, from ab- sence of the frontal processes, the two orbits form a single cavity, and the eyes are more or less blended in the mesial line. See JIon'STROS- ITY. The following is a brief summary of the suc- cession of events that occur in the ordinary or normal development of the skull. Cartilage is formed at the base of the membranous capsule, which has been already described as thrown round the brain and capable of enlarging with it. This is speedily followed by the deposition of osseous matter at various points of the cap- 2, poste- ,.w. ..... ... 3. sagittal suture: a continuous and 4, 4, coronal suture; 6, 5, lambdoid ultimatelv an un- suture; 6. 6, parietal bones; 7, 7. vipldiiif bnnv cn^p two halves of the frontal bone, still } leioiiig oon ca^e ^united; 8, occipital bone, adniirablv 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 diploic, 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, but 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 pericranium externally, through numerous forn- niina. 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 fossae, corresponding to the anterior and middle lobes of the cerebrum and to the cerebellum. These fossfe are marked, as is the whole skull-cap, by the cerebral convolutions, and the.v 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 vomer; the