The section last described completes what may be called the theoretical structure of the tongue (it is shown in fig. 748., in one half of its extent) : the next change is one of in fraction of that completeness of muscular structure, occasioned by the entrance of the genioglossi muscles on the inferior surface, whereby the continuity of the cortical layer is isolated, there being no discs in the space occupied by the immergence of these two mus cles. The subsequent changes in the appear ance of successive sections, are just such as might be expected from the description al ready given of such sections when viewed by the naked eye ; therefore, for the sake of brevity, I shall not here advert to them.
A longitudinal vertical section, of course, displays a reversing of the position in which the discs and profile interlacement are re spectively seen ; the discs are here in the central part instead of in the cortical, being the transverse fibres cut across ; the cortical layer is free from discs (except where, from the vertical and longitudinal fibres being ob lique, they are obliquely cut), and is occupied by the longitudinal and vertical fibres, both seen in profile.
A horizontal section displays very much the same appearance as the last : the discs are in the central part, and there is an absence of them in the circumferential portion; but in this case the discs are of the vertical fibres, in the other they were of the transverse.
Superficial sections, or sections made ob liquely, I shall not attempt to describe ; by such means the appearances might be in finitely varied, but their description would merely tend to confusion.
From the above account it is seen that the microscopical investigation of the subject not only confirms and proves the conclusion ar rived at by other means, but adds many new and interesting facts, and supplies us with one more instance of that contrivance and adapta tion of means to end that meets us at every point.
I have entered rather fully into the intrinsic muscular arrangements of the tongue, because there is no good account of the subject, that I can find, in the English language, nor of the appearances as seen by the microscope, in any language ; and I considered such an account a desideratum.
Mode of termination of the intrinsic fibres. — All the intrinsic fibres of the tongue, and indeed, it may be said, almost if not quite all of the extrinsic too, terminate by becoming in serted into the cutis — the sub-mucous fibrom tissue — which is extremely dense and thick, particularly on the upper surface. The trans verse and vertical fibres pass direct to the cutis ; the longitudinal all ultimately have a similar insertion. In vertical sections the fibres
may be seen passing up or down to the sur face, and entering the cutis, which having pierced fora certain extent, they terminate :— some of them end just as they enter the fibrous tissue ; some of them may be traced quite up to the papillae.
It might naturally be expected that the muscular fibres of the tongue would, from their isolation, present great facilities for see ing the mode of the termination of muscle in fibrous tissue ; and indeed this is the case in a remarkable degree; they seem to furnish the great desideratum of a natural isolation of the individual fibres, at the point where you are sure of seeing their union with the fibrous tissue that forms their means of attachment. As the opinions that I have come to from my own observations differ from those that are the result of some of the best researches on the subject that have been made, namely those of Mr. Bowman, as published in the "Philo sophical Transactions," and in the " Physio logical Anatomy " of himself and Dr. Todd, I would advance them with diffidence, and some thing like hesitation. The appearances that I have invariably seen, by daily examinations for some time, of sections from many tongues, is this: each fibre, before its termination, gradu ally tapers, in a fusiform manner, with more or less of acuteness ; sometimes the tapering is rather sudden, in other cases very much pro longed (fig. 752. A) ; in all the tapering is con tinned so far that the muscular tissue becomes nearly as fine as the fibrous tissue in which it terminates : from this fine extremity the fibre passes off, and the appearances at the point of transition are of two sorts ; the one as seen at a, a, where the muscle passes smoothly off in the fibrous tissue, and you cannot tell where the one commences and the other ends ; the other as seen at a, b , where the pointed extremity of the muscle is a little rounded, and its outline plainly visible ; but in this case, also, the diameter of the extremity of the muscle as nearly as possible coincides with that of the fibre. It is possible that the dif ference of appearance may depend in some degree upon difference of focus, but, certainly, I have not been able, in some cases, by any adjustment of focus, to get a clear definition of the point where the muscular structure termi nated. In those cases where the outline of the termination of the muscle is defined, the transverse striation may be traced up to its very extremity ; where the outline of the one merges in that of the other, the striae seem also gradually to be lost, becoming a linear series of little dots, and so fading away (a, a).