By removing the bones of the face and taking the sphenoid in conjunction with the frontal bone, we shall (if we place the body of the sphenoid bone vertically) at onoe per ceive the same analogy to exist. If, when they are thus placed, we look at the cerebral surface, we shall recognize the body in that of the sphenoid ; the vertebral plates in the small wings of the sphenoid, and two halves of the frontal bone ; the foramen in the space cir cumscribed by these last ; the transverse processes in the two great wings of the sphe noid; and the notches in the lacerated orbitar foramina, and the angles between the body of the sphenoid and posterior marain of its great wings. If we look at it in front, it will not require any great stretch of the imagination to recognize the four articulating processes in the pterygoid processes of the sphenoid bone and the external angtilar processes of the frontal.
The temporal and the parietal bones toge ther represent another vertebra, situated be tween the former two. By looking at the base of the skull held vertically, .and abstract ing in tbe mind the occipital bone, we can (under favour of the license allowed to, or taken by anatomists) see in the two petrous portions of the tempoml 'bones, if they were brought into contact, a type of the body of a vertebra; and in those parts of them which contribute to form the anterior and posterior lacerated foramina, we observe a resemblance to those notches which form in the vertebree, as they do here, conjugal fommina. The arti cular eminences of the temporal bones give us no bad notion of the transverse processes, while the zygomatic processes above (still bolding the skull vertically) and the part which projects behind the mastoid processes below, will indicate the four oblique or arti culating processes. Lastly, the squamous pro cesses of the temporal and the whole of the parietal bones represent the vertebral plates, and the space enclosed by them, the vertebral foramen.
Development of the cranial bones.— The progressive development of the bones of the cranium has been pointed out in their separate descriptions ; but there are some general facts which regard its formation as an entire organ which merit further notice.
The cranium of the fcetus presents, like all other organs, a rude outline of the shape it is destined to assume; and, at the earliest pe riod at which it is noticed, its walls are com pletely membranous, being formed by the dura mater and pericranium so united as to render it impossible to separate them without injury. Very early points of ossification are developed in this membranous envelope, whence osseous radii shoot out, so that the several points enlarge towards each other, and ultimately coalesce or are united by suture.
Unlike other bones of a similar character the opposite surfaces are not of similar den sity. The surface secreted by the vessels of the dura mater contains less animal matter than that which is produced from the vessels of the pericranium ; and it is, therefore, of a more dense and brittle character; so much so, that, when the contig,uous bones approximate, the edges of the inner table are simply in juxta-position, a slight layer of cartilage alone separating them. Hence, in the interior of the skull, the sutures are plain lines ; or, if at all irregular, there is no interchange of substance between them. Not so, hoi,vever, with the external. By reason of the greater quantity of animal matter which it possesses, and the more diffuse character of its texture, a prin ciple of toughness is conferred on it which admits of its being dove-tailed with the same table of other bones.
The base takes precedence of the calvaria in the commencement and completion of its ossification. With the exception of its most prominent points and the ethmoid bone, it is completely ossified at birth; while, between the bones of the calvaria, there are conside rable membranous interspaces, so as to allow of these bones being squeezed together, or to overlap each other, at the period of parturition. The ossific matter departing from the pro tuberances of the frontal and parietal bones (c,d,figs. 374,375) and radiating to wards the circum ference of these bones, it follovvs that the angles will be incomplete when the rest of the bone is formed.
On this account it is that, at the four angles of each pa rietal bone, there is a membranous spot which the ossific matter has not reached, when, in other parts, it is joined to the surrounding bones. These spaces are called.fontanelles ; two of them are situated on the median line and superiorly ; and two others inferiorly and in each lateral region. The posterior superior fontanelle is triangular and is found between the superior angle of t'he occipital bone, and the occipital angles of the two parietal. The anterior superior fbntanelle (a, fig. 376), by reason of the frontal bone being formed in two parts, is of a lozenge shape ; and it is between those two parts and the frontal an gles of the parietal bones that it occurs. These two fontanelles are conse quently at the extremities of the sagittal suture.