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CONSOLIDATION OF CRANIAL WALLS.
Whilst the bones of the extremities are constructed for support and mobility,—those of the trunk for support, mobility, and the protection of their contained viscera,—the cranium, regarded in its primary idea, seems essentially designed for affording security to the delicate and important organ it encloses; for which object there is a steadily advancing process of development, continued through the earlier and middle periods of life, until it may be said to have attained its final form or a state of perfection ;-that is, when all the separable portions of which it consisted in youth have become consolidated into one bone.
In consequence of this progressive change, the anatomy of the skull necessarily varies with the different periods of life at which it is examined. What is at one time an insulated mass of bone becomes at another and more advanced period firmly united, and continuous with the portions surrounding it. On taking, for example, the cranium at the commencement of extra-uterine life, the frontal bone consists of two, and the occipital of four distinct or discontinuous portions ; whilst at a later period these become respectively consolidated into individual bones, each presenting an uninterrupted extension of osseous structure. Again, to take the cranium at a still later period, the osseous masses that are conventionally regarded as separate cranial bones, become themselves united and consolidated with each other. The sphenoid and occipital bones, for instance, in early youth, exist as separate and independent segments ; but at a more advanced age, they become so firmly united together at the basilar process, as to require artificial division with the saw, even whilst the other segments of the cranium are still easily separable from
UNION OF SEGMENTS INTO A CRANIO-FACIAL BONE.
each other. The sphenoid and occipital, in fact, at this period constitute as much a single bone as any of the rest, and might consequently not unjustifiably be denominated the spheno-occipital bone.
It is therefore evident that the generally received anatomical division of the cranium into a certain number of bones is an artificial and arbitrary separation, corresponding only with a very transitory period of human existence ; and that during the progression towards consolidation (which is steadily but constantly advancing), no precise anatomical division can be made which shall exactly coincide for two separate or distant periods. Although, therefore, I shall incidentally allude to the separate bones for the purpose of enabling you to identify the parts of the skull to which I am referring ; yet it is
my intention to advert to the anatomy of the cranium at that period of life—the adult period—when all the separable portions of which it consisted in youth have become consolidated or united into one mass, that may not unaptly be termed the cranio-facial bone ; and when it may be thus regarded as having attained its most perfect condition. A precedent for such an arrangement may be found, in the course that is adopted by anatomists with regard to the description of the os innominatum. This bone, although with the attainment of adult age it becomes consolidated into one mass, yet the separate parts of which it was formerly composed are still individualised by retaining the distinct names-ilium, ischium, and pubes—that were attached to them when they existed as independent and isolated bones.
The order of this progressive union or consolidation of the cranial segments seems to bear an intimate relation to the completion or development of the con
CONSOLIDATION : RELATION TO GROWTH OF BRAIN.
tiguous portions of the brain. The osseous structure thus attains its most efficient condition for carrying out its primary and especial design-security ; cotemporaneously with the completion or perfection of the organ, which it is intended to protect.
It is, if we reflect on it, a necessary condition that consolidation should not be completed till such time as the growth of the brain shall have attained its maximum. For as the cranial bones grow through the medium of the membranous structure that divides them ; so soon as complete union is effected, no further increase in the capacity of the cranium can take place. But, as soon as the cerebral structures have attained their full development, no further increase of capacity is required, and the bones become consolidated to afford the requisite protection and security to the extremely delicate organ they invest.
This gradual consolidation of the cranial case forms an exceedingly striking and beautiful arrangement, when viewed more attentively, in relation to the growth and development of the brain. In very early life, whilst the brain is undergoing a very rapid growth, there is a correspondingly rapid increase required in the extension of the cavity of the cranium. To effect this, the cranium grows through the medium of a great number of individual and isolated centres, each of which, spreading itself out in certain and appropriate directions, produces a very rapid increase in the general dimensions of the cranial cavity. As the demand for this rapid extension becomes diminished, by a more gradual development of the brain, the number of individual osseous centres of growth becomes proportionately decreased by the union or consolidation of contiguous portions of
NUMBER OF SEPARATE SEGMENTS : THEIR OBJECT.
bone. Thus, to take again as examples, the frontal and occipital bones; during the earlier periods of the development of the brain, when it is increasing rapidly in size, the former consists of two, and the latter of four, distinct and isolated portions, each portion forming an independent centre of growth. These independent and isolated masses afterwards unite, so as to form single bones ; and the same consolidating pro
; cess continuing between the separate bones themselves, the power of extension of the cavity of the cranium becomes gradually diminished, until at last, when the complete development of the brain has been accomplished, and its maximum size has been attained, no further increase of cranial capacity is required, and the bones become consolidated into a bard unyielding case, incapable of any further normal expansion.
That the true object of the great number of isolated portions or centres of bone, observed in the foetal skull, is to produce a rapid extension of the cranial capacity, cotemporaneously with the rapid growth of the brain at this period, is fully confirmed by the appearances presented by the cranium, in cases of hydrocephalus. In these unfortunate subjects there is a demand for a greatly abnormal and comparatively rapid expansion of the cranial cavity, which, in a considerable measure, is effected by the large development of numerous islets or insulated masses of bone, known in anatomy as the ossa Wormiana. Each of these, growing from its own centre, produces such a rapid extension of surface that, until consolidation takes place, the cranium is adapted to the rapidly increasing bulk of its contents.
Amongst the osseous portions of the cranium, that are regarded and described by anatomists as constituting separate bones, the sphenoid and the occipital
6 REASON OF EARLY UNION OF SPHENOID AND OCCIPITAL.
are those that become first consolidated or united together. But this consolidating process, which here commences between what are conventionally called the cranial bones, is only a continuation of that which has been already taking place between the previously separate portions of the bones themselves; and which, still proceeding, finally solidifies the cranial case into a continuous, unbroken, or uninterrupted layer of osseous tissue.
The reason of this early union between the sphenoid and occipital seems probably connected with the following considerations. Being situated in the centre of the skull, their timely consolidation is required to enable them, in acting as a central or fixed point, to determine the
proper direction of growth of the surrounding cranial bones. And their line of union being seated at the basilar process, the period of their consolidation presents also a bearing in reference to the maximum growth or full development of the medulla oblongata. This portion of the nervous centre, although simple in its function, yet, being associated with the processes of respiration and deglutition, is most essential to the persistence of animal life. It consequently early attains a state of completion, as regards its growth and development, and therefore correspondingly early necessitates a state of completion or of consolidation of the portion of osseous structure over which it rests.
The limits of these Lectures will not allow me to attempt to trace to you the line of order in which the union of the rest of the bones takes place. Such order is, I have no doubt, determined by definite physiological relations, a careful investigation of which would afford an exceedingly interesting subject of inquiry.
Having thus cursorily directed your attention to the gradual consolidation of the walls of the cranium, and