SKULL. The skull is divided into two part's, the cranium and the face. In human anatomy, it is customary to describe the former as consisting of 8 and the latter of 14 bones; the 8 cranial bones, which constitute the twain-case. being the oceipita/, two parie 1(4 frontal, two temporal, sphenoid, and efh maid; while the 14 facial hones are the two vasal, two sniwrior maxillary, two lachrymal, two molar, two palate, two inferi:,r binated, vomer, and inferior maxillary. The bones of the ear, the teeth, and the Wormian hones are not 'included in this enumeration. The morphologist, however, who wishes to trace out the fundamental similarity of type in the structure of the various modifications of the vertebrate skull, will content with this arrangement, iv which, as, for example. in the occipital, temporal, and sphenoid bones, the human anatomist consid ers as a single bone an osseous mass consisting primarily in man, and persistently in some of the lower vertebrates, of several distinct pieces or elements. Postponing to the close of this article any remarks on the structure of the vertebrate skull generally, we shall proceed to the ordinary anatomical relations of the human skull. The. de velopment of the skull is a subject of great interest, not only in itself, but as throwing light on many points which the study of the adult skull would fail to explain. a very early period of fetal existence, the cerebrum is inclosed in a membranous capsule exter nal to the dura mater, and in close contact with it. This is the first rudiment of the skull, the cerebral portion of which is consequently formed before there is any indica tion of a facial part. Soon, however• four or five processes jut from it on either side of the mesial line, which grow downward, incline toward each other, and unite t-o form a series of inverted arches, from which the face is ultimately developed. Imperfect development or ossification of these rudimentary parts of the face gives rise to the peculiarities known as "hare-lip" and •• cleft-palate," or in very extreme cases to the form of monstrosity termed " Cyclopean," in which, absence of the frontal processes, the two orbits form a single cavity; and the eyes are more or less blended in the mesial line.
The following is a brief summary of the succession of events that occur in the ordi nary or norm.] development of the skull. Cartilage is 5°r-tiled at the base of the mem branons 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 ossific mat
ter at various points of the capsule, which soon becomes converted into flakes of bone, which afford 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 flakes on the convexity of the cranium is soon followed by the appearance of oaseous nuclei in the cartilage at the base, corresponding to the future occipital and sphenoid bones. Lastly the various bones. some in membrane, and some in cartilage (as described in the article Osswicknox). approach one another by gradual enlargement, and become united in variuous ways, so as to form a eontinons. and ulti mately an unyielding bony case, which, in the words of Dr. HumpItrey, "is admirably adapted for th- defense of the brain, for the aceonunodation of the organs of special rose, and for the attatelonent of the ligaments and muscles by which the skull is sup po•ted and moved on the spine."— The Hermon Skeleton, p. 1 At the period of birth most of the principal hones have e•own into apposition with their neighhors, forming the ..uteri's (q.v.), but one large vacaity remains at the meeting-point of the parietn1 and frontal bones which is termed the font:mac,* which does not •loFe till the sec ond year after birth. and sometimes remains open much longer. The deficiency of the osseous brain-ease at this position not only facilitates the act of delivery, but also acts, according to Humphry, to some extent like a safety-valve during the first months of infantile life, at which time the brain bears an unusually large proportion to the rest of the body, and is liable to sudden variations of size from temporary congestion, sude,en wasting of its substance, and other causes. The sutures remain distinct long after the closure of the fontanelles, and probably serve a purpose both in permitting an increase of the size of the cranium by the growth of the bones at their edges(although the enlarge ment of the cranial cavity does not entirely depend upon this growth at the edges), and in diminishing and dispersing vibrations from blows, and thus contributjng to the secu rity of the brain.