Since the longitudinal fibres invest the whole of the free surface as a sheath ; since they are, most of them, not directly, but obliquely, longitudinal ; and since many of the central spread out to the sides, while the lateral converge to the centre, the division of the longitudinal linguals into superior, lateral, and inferior must be to some extent arbitrary : however, most of those on the upper surface are intrinsic in their origin, those at the sides are extrinsic in their origin, while those on the inferior surface are sufficiently individual and distinct: some subdivision appears ne cessary, and the one adopted will at any rate assist in remembering these facts.
The microscopical examination of thin sections.
On making a thin transverse vertical section of the human tongue, and examining it with the microscope*, we see that the appearances indicated by a similar section, viewed with the naked eye, are correct, namely, that the in trinsic muscular fibres assume three principal directions, a vertical, a transverse, and a lon gitudinal ; and that the longitudinal are con fined to the neighbourhood of the surface. But we see more; we see a very curious and artificial arrangement of the fibres very much contributing to facilitate their package, and by which they mutually support one another and act with the greatest advantage. This arrangement I shall now proceed to describe. Suppose the section made at a point just in front of the anterior free border of the genioglossi (fig. 748.). Immediately beneath the papillw (which may be very well dis played by this method), the condensed sub mucous areolar tissue or cutis of the tongue is seen, of considerable thickness, being thickest on the upper surface, especially towards the middle (aa). Immediately beneath this, around the whole circumference of the tongue, is seen a very curious arcolated or fenestrated ap pearance, consisting of cross bars, branching and interlacing irregularly at various angles, leaving interspaces that are filled up by groups of discs (dd). The cross bars are at once seen to be small fasciculi of the vertical or trans verse fibres, or both, according to the part looked at, and the groups of discs are seen to be transverse sections of the longitudinal fibres passing through the meshes formed by the vertical and transverse, which they more or less completely fill, and with whose shape they more or less exactly correspond (figs. 748. and 751.). The fasciculi of the longitudinal fibres are in most situations much larger than those of the vertical and transverse, among which they are contained ; indeed, the lon gitudinal being confined to the surface, it would naturally be expected that they would preponderate there. The vertical fibres are most abundant in the vertical median plane and the horizontal in the horizontal median plane (f), the vertical not existing near the lateral surfaces, nor the transverse near the superior and inferior surface (figs, 749. a, a); and from this fact result almost all the pe culiarities of arrangement of the fibres that we see.
In the first place it results from this, that the vertical and horizontal fibres cross each other in the centre, which they entirely occupy, and therefore exclude the longitudinal ; ac cordingly no discs are seen in the central part of the tongue. Secondly, that at a cer tain line (fig. 748. g.) the vertical emerge from the transverse, and are continued up or down, to the superior or inferior surface, alone; and similarly at the lateral regions the transverse emerge from the vertical, and are continued on alone to the cutis at the sides ; hence the fibres near the middle of the upper and under surface, and at the borders of the tongue, do not interlace but pass to the surface with something of parallelism ; and hence the fasciculi of longitudinal fibres here are arranged, not as in the mesh of a network, but in parallel rows at right angles to the surface ; an arrangement very charac teristic of these situations. Thirdly, it would
result from this absence of vertical fibres at the sides, and of the transverse above and below, that there would be four situations (b, b, b, b, fig. 749. A .) in the neutral ground between the upper and under surfaces and the borders respectively, where there would be no cross fibres of any sort, and where the lon gitudinal fibres would exist alone, unsupported and unseparated. Moreover, the vertical fibres at the upper and under surface, and the trans verse at the sides, would be so dense and numerous that they would hardly admit of any longitudinal fibres in their interspaces. Now the support and separation of the longitudinal fascicuh, and the admission of a sufficient number of them at all the superficial parts of the tongue (especially the two surfaces and the two edges, which may be called the cardinal points of the tongue with regard to its move ments), are the two things that are especially to be brought about. To achieve this double object, the vertical and horizontal fibres, as they approach their respective surfaces, spread out in a sort of fan-like manner ; the most lateral of the vertical fibres spreading out to wards the sides (fig. 748. k, k), and the most superior and inferior of the transverse spread ing up and down towards the surfaces (fig.748. i, i, fig. 749. B.). The two sets thus cross each other and fill the otherwise empty space with a network of considerable regularity and beauty, which is characteristic of these four situations, as the parallel fasciculi at right angles to the surface are characteristic of the fbur inter mediate ones. For the sake of convenience I shall call the situations where the transverse and vertical fibres approach the surface in pa rallel bundles, the superior, the inferior, and the right and left lateral regions (fig. 749. A, B. a, a, a, a): those in which they decussate as they approach the surface, I shall call the right and left supra-lateral, and the right and left sub-lateral (fig. 749. B. b,b,b,b.). Fourthly, the mesial vertical and mesial horizontal plane are the situations where the vertical and hori zontal fibres respectively would act with the greatest power on the form of the tongue, and where also they would admit of being the longest; hence we see the fasciculi in these situations much larger and more densely packed than in the intermediate positions, so that they more than equal the longi tudinal fibres that they transmit. The ex treme lateral fibres, on the other hand, that spread out and interlace, having little more for their office than to support the longitudi nal fibres, are very small and scanty, many of them consisting of only a single fibre, and hence at these points the preponderance of the longitudinal over the vertical and hori zontal fibres is the greatest. Fifthly, the most deep-seated of the longitudinal fibres of the upper and under surface are underlaid by a definite floor of transverse fibres, and simi larly the deepest of those at the side are under laid by a floor of vertical fibres; therefore in these situations there is a strong line of de marcation, the discs are abundant down to the bottom of the cortical layer, and there they terminate suddenly (fig. 7.51.); but in the in termediate positions there is no definite floor, no line of demarcation, but the discs of lon gitudinal fibres dip down at irregular distances (figs. 748, 749, 7.50.).