Relations. — Its whole external surface is devoted to muscular insertions, which separate it, anteriorly and laterally, from the cutaneous structures. Behind, it is in relation with the epiglottis, from which it is separated by some dense cellular tissue, and by the thyrohyoid membrane, and with the mucous membrane of the pharynx.
In man the hyoid bone consists of five pieces, —the basis or body, two greater cornua which project backwards from the sides of the body in a direction nearly horizontal but a little upwards, and two lesser cornua, sur mounting the body and greater cornua at the point of their union.
The body is quadrilateral, compressed an tero-posteriorly, curved, and laterally ex tended. Its anterior surface looks upwards, is convex, and intersected by two irregular ridges, a vertical and horizontal, which at the point of their union project in a prominent tubercle, which is the analogue in man of an additional element of the hyoid apparatus in the lower animals—the true lingual bone (glossohyal): this tubercle is sometimes bifurcated. The portion above the transverse ridge is directed much more upwards than that below, so that it has sometimes been called the upper sur face : in that case the portion below is de scribed as the whole anterior surface, a cir cumstance which has led to much confusion with regard to the nomenclature of muscular insertions into this bone. The posterior surface is concave, and looks downwards ; it is marked by many little foramina entering the cancellous structure, and is sometimes covered by a syno vial membrane. The lateral surfaces, small and articular, are connected to the anterior ex tremities of the greater cornua by a lamina of temporary cartilage, which, however, is seldom completely ossified, and which admits a certain amount of movement between the different parts. The superior border presents a double curvature, something like the upper lip of the mouth, i. e., it curves downwards at its ex tremities, and has a little dip in the middle. The inferior border, of less extent, is thinner, horizontal, and placed on a plane anterior to the upper.
The greater comae are at least half as long again as the body, from the sides of which they project, at first a little outwards and then backwards; they possess a shaft and two extremities. The anterior extremity is large, club-shaped, tuberculated, and curved in wards towards the body, for its attachment with which it presents an articular facet ; the shaft tapers gradually posteriorly, and is la terally flattened, so that it has an outer and inner surface, and an upper and lower border. Its outer surface, which is continuous with the anterior surface of the body, looks a little upwards ; its lower border is smooth and rounded ; its upper border sharp, and, from the obliquity of the surfaces, of less extent than the lower. The posterior extremity is expanded into a little tubercle, sometimes sur mounted by an epiphysis.
The lesser cornua (ossa pisiformia lingualia, of Stammering) are two little pyramidal or pisiform nodules, projecting upwards, out wards, and backwards, from the point of union of the greater cornua and body : they are seldom completely ossified. They are the homologues in man of a very considerable process (the ceratohyal) in the lower animals, in some of whom the proportion between the lesser and greater cornua is inverted.
Structure — Chiefly compact, but a little cancellous in the body and large extremity of the greater cornua.
Development. — From five points ; one for each element. Vesalius saw a case in which there were six, there being two for the body. The ossification commences in the greater cornua ; it then takes place in the body, where it begins soon after birth ; and, finally, in the lesser cornua, where it does not com mence till some months after. It proceeds but slowly, and generally leaves a thin lamina of cartilage unossified, so that complete an= chylosis of the different parts into one bone is comparatively rare.
The morphological value of this bone and its homological relations, will be treated of hereafter.
b. The hyoglossal membrane or ligament. — This is a vertical transverse lamina of very dense areolar tissue, containing a large pro portion of the yellow element, passing up wards from the upper border of the body of the os hyoides to the tongue, between which it constitutes a means of union. The mus cular connection of the tongue with the hyoid bone by means of the hyoglossus is deficient in the central part of the body of that bone, and, consequently, this ligament is, in that situation, the only direct bond of connection. It is dense, yellow, and very elastic, and has a little fat dispersed among it, though this is denied by Bichat. It may be traced upwards into the tongue, sometimes as fir as an inch. It receives on its anterior face those fibres of the genioglossus that lie immediately above those that are inserted into the hyoid bone, and also some of the intrinsic longitudinal fibres of the tongue that terminate upon it ; posteriorly it is in relation with the upper part of the epiglottis and the mucous membrane reflected upon it from the tongue ; in fact, it immediately underlays the glosso-epiglottid folds. Above, it is gradually lost in the muscles of the tongue ; below, it in part terminates in the upper lip of the body of the os hyoides, and in part is continued on behind this bone, constituting the yellow elas tic tissue already referred to as being inter posed between it and the epiglottis. This ligament has been well described by Bichat.* e. Median fibrous septum (median carti laginous la»zina, Blandin). — Springing from the anterior surface of the last-mentioned structure, interposed between the two ge nioglossi muscles, passing forwards between these two muscles as far as their genial origin, upwards to the dorsum of the tongue as far forwards as its centre, and thence to the anterior free border of the genioglossi, and a little beyond that border, is a vertical lamina of fibrous tissue. It is thick and dense behind and below, but gradually becomes thinned out as it spreads upwards and for wards; as it gets thinner it becomes cribriform, like the septum of the corpora cavernosa penis, the areolm giving transmission to the transverse muscles of the tongue, which pass through it from side to side. It varies much in different individuals ; in some it is tolerably dense, in some it is merely a fine areolar web. This structure M. Blandin has dignified with the name of" median cartilaginous lamina," and has described it as consisting of a vertical sheet of that substance of more or less extent. I have, however, looked in vain for any thing like cartilage or fibro-cartilage in any part of it. It appears to me to be nothing more than intermuscular areolar tissue in rather greater amount and density than usual, which hap pens to be placed in the median plane. It has been supposed by some to be the analogue of the small fusiform slip of cartilage placed beneath the extremity of the tongue in the dog and wolf, with which, however, it has no relation : that structure is essentially dis tinct. Others have found in it the analogue of the lingual bone, a conception still more far-fetched. It has been supposed to give origin to muscular fibres by its two surfaces. This I have failed in detecting ; certainly it does not give attachment to any of the trans verse fibres of the tongue, to which it might be supposed it would, if to any; for they may be seen, by transverse sections viewed with the microscope, to pass, without excep tion, from side to side of the tongue, without any break in their median plane.