ARRANGEMENT ADOPTED IN THESE LECTURES.

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the relation it bears to the progressive growth and development of the brain; let me next proceed with an investigation into the nature and intention of the various processes which give to the skull its extreme irregularity of surface and complexity of configuration. If we contemplate for a moment on the delicate structure of which the cerebral organ is composed; and then glance at the hard, rough, and irregular surface the internal cranial base presents; it certainly would, at first sight, suggest the idea as being far more adapted for injury than for protection to the brain. But if we pause and examine more closely into the relation which these projecting processes bear to the other parts of the cranium, we shall, I think, not only be struck with the beauty of their arrangement, but be convinced of the salutary influence they exert in maintaining the integrity of the brain during the exigences and necessities of everyday life. Rely upon it as an indisputable fact, that nothing in the cranium exists unless designed to carry out some definite intention, or unless formed in reference to some distinct and special object.

In pursuing this inquiry, and adopting, as far as is practicable, some systematic arrangement, I shall engage your attention, first, with the exterior and then with the interior of the cranial vault; next with the internal, and then with the external base; and shall afterwards, before proceeding with the bones of the face, advert to those exceedingly interesting changes which are brought about by the final completion or the full development of the sphenoid bone.

Between the superciliary ridges of the frontal bone of the adult skull, is a more or less well-marked prominence, termed the nasal eminence, which corresponds

with and is produced by the development of the frontal cells or sinuses. These sinuses constitute accessory cavities to the nasal apparatus, and are formed at that particular period of life when the sense of smell is attaining its highest state of functional activity. They remain entirely undeveloped during infancy and early youth; and it is my impression, from a careful examination of the numerous skulls belonging to the Hospital Museum, that they do not commence to make their appearance until fourteen or fifteen years of age, or even in many instances at a still later period than this. They certainly are not developed at anything like so early an age as is usually credited. Here, for example, is the skull of a youth, as near as I am able to ascertain, of about thirteen years old; and from the section that has been made, you will observe there is not yet the slightest vestige of a frontal sinus. Here is the frontal bone of another skull, which from the historical data that are attached to it, and from the source whence these data were procured, may be confidently looked upon as belonging to the age of eighteen. A portion of the anterior plate of bone, between the superciliary ridges has been removed, and the frontal sinuses are observed in an exceedingly incipient or rudimentary condition. Passing still onwards, in point of age, we have before us a third skull; that of an adult, of about twenty-two years of age. The frontal cells have been exposed in a similar manner to the last, and, as you notice, present the appearance of considerable sized cavities. I believe, however, that they do not remain stationary even at this age; but that they still continue enlarging until a period of fully-attained maturity of adult life.

IRREGULARITY IN FORM OF FRONTAL SINUSES.

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From the undoubted functional connexion of these sinuses with the nasal organ, it is not difficult to understand why they should be absent in early life, nor to appreciate why, at a later period, they should be developed in anticipation of those higher functional endowments, which the nose acquires an an olfactory and respiratory organ. But these sinuses do not form the only accession that is made to the nasal apparatus, whilst it is thus acquiring an increased state of perfection as an organ of smell, for there are other cavities connected with the nose, namely, the antrum of Highmore and the sphenoidal cells, which, like the frontal sinuses, remain undeveloped during early life, when the functional capacity of the olfactory organ is comparatively less delicate and acute.

On examining different specimens of adult crania, the extreme diversity or irregularity observed with regard to these sinuses, constitutes an exceedingly curious and striking object of consideration. Sometimes they are altogether absent; occasionally there is but one solitary cell; oftentimes there is a single small cell on each side, whilst at other times they form fair-sized cavities-in some instances symmetrically disposed on the two sides of the median line, but more often exceedingly irregular both in outline and dimensions. They are sometimes so largely developed as to extend upwards for nearly the whole distance of the forehead, and likewise backwards for an inch, or even more, along the horizontal plates, constituting the arches of the orbits.

At Plate I. there are six sketches, which show the great variety of aspect these sinuses present. They have been taken from horizontal sections of the frontal

bone, made through the centre of the nasal eminence, and the sinuses are seen as though looking down into them from above. In the upper specimen there are none. In the second there is a solitary cell on the right side. In the third there are two small cells, one on each side of the median line, whilst in the other three, the cells are of much larger size those in the last being, at the same time, extremely irregular in outline.

The frontal cells are usually described as divided into two lateral sets by a vertical septum of bone. This septum, as you may observe from the specimens on the table, is sometimes complete, and then separates entirely the sinuses of one side from those of the other, but is likewise often incomplete, so that in the dry preparation the two sets of cells communicate laterally with each other. It must not be regarded as a necessary consequence, however, because the osseous septum is imperfect, that in the recent preparation or in the living subject, the sinuses of one side should be directly continuous with those of the other. For instance, in the specimen before each lateral group of cells is lined by a separate membrane, prolonged from its respective side of the nasal cavity, and these two membranous extensions, becoming adapted in the median line, complete the separation where the osseous septum is imperfect.

you

The drawing at Plate II. represents the condition of the specimen adverted to. It is a vertical median section of the frontal sinuses in the recent preparation, and shows the membranous extension completing the separation where the median osseous septum is imperfect.

The process by which the frontal cells are developed constitutes a striking illustration of the perfection of

MODE OF FORMATION OF FRONTAL SINUSES.

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the designs of Nature; as we are able to trace their formation to the same cause which is increasing the efficiency of the organ to whose function they contribute.

As I have already stated, they do not appear until after a much later period of life than is generally allowed. Before the commencement of their development, the two plates or tables of bone, which constitute the inner and outer surfaces of the cranial case, are placed in close apposition to each other. But the outer plate, being now pushed forwards in a manner I shall directly point out, by the central lamella of the ethmoid under the influence of the development of the sphenoid, and the inner plate retaining its original position in relation with the crista galli and cribriform plate of the ethmoid, the intermediate structure or diploë is extended into cells or cavities varying in size according to the extent to which this process has advanced.

I must needs, for further explanation, anticipate a little of what I shall hereafter have to say concerning the development of the sphenoid. The final completion of this bone does not take place until after the completion or development of the remainder of the cranial bones; and wedged in, therefore, as it is into the immediate centre of the cranial base, its progressive growth or expansion produces vast and important changes in most of the surrounding parts. The body or centre of the bone is, in early life, solid and comparatively small in size, but it afterwards becomes hollowed out into cells, which, like the frontal sinuses, form accessory cavities to the organ of smell. With this development of the sphenoidal cells, the body of the sphenoid is proportionately increased in dimensions, and the rostrum-a process of bone projecting from its inferior