d. Lastly, the investments of the tongue, in cluding the papillary structures and the true cutis underlaying them, may he looked upon, in one light, as a part of the framework of the organ. They form a dense and unyielding envelope, tending to preserve its shape and give it firmness and support, and at the same time affording attachment to a great number of its muscular fibres.
Such, then, is a concise description of the framework of the tongue. The first and last mentioned elements of it are, doubtless, the most important; but it will be seen, at a future page, that the intrinsic arrangement of the tongue's muscular fibres is such that they mutually support each other, and tend to keep the organ firm and compact, which obviates the necessity of any considerable structures especially destined for that purpose.
2. The muscular system. — Constituting the chief bulk of the tongue, imparting the required consistence to it, performing the ma jority of its functions — prehension, mastica tion, deglutition, speech,— and necessary even to the perfection of taste, the muscular sys tem of the tongue may he considered the most important of all.
The muscles of the tongue are of three sorts, and admit of the following arrange ment : a. Intrinsic.
b. Extrinsic.
c. Accessory.
The intrinsic muscles are those which form the substance of the organ, that pass from part to part of it, and that move the tongue on itself.
The extrinsic (proper) are those that, as well as entering in some degree into the sub stance of the tongue, pass from it to neigh bouring fixed points, to which they attach it, and on which they move it.
The accessory are those which, though not contributing in any degree to the formation of the tongue, nor attached to it, are yet engaged in all its extrinsic movements, acting as coadjutors to those proper extrinsic mus cles whose direction coincides with theirs.
Of the two first I shall speak particularly : little more than their enumeration will suffice for the last.
a. Intrinsic muscles of the tongue. — There are three methods of investigating the arrange ment of the tongue's intrinsic muscular struc ture : first, by the ordinary method of dissec tion, or separation ; secondly, by making sections in different planes, and examining the appearance of the cut surfaces ; thirdly, by the microscopic examination of thin sec tions. The first, which is the oldest, is that by which we gain the least information ; and to its adoption must be attributed the fact that so many of the older anatomists were in the dark on this subject. By it we merely learn the following facts : that certain of the ex trinsic muscles pass into the substance of the tongue and contribute to certain of the intrinsic ; that the direction of most of the superficial fibres is more or less longitu dinal ; that the direction of the more deep seated fibres is not longitudinal ; that it is complex, and incapable of demonstration by separation.
The second method furnishes much more certain information. By making the sections in different planes, we vary the point from which we regard them ; and the section made in one plane corrects, and supplies the defi ciencies of, the other. The transverse vertical section is the most important ; and to this I shall chiefly refer.
On making a transverse vertical section of a human tongue, at a point just behind the ante rior free margin of the genioglossus ( fig. 747.), the following are the appearances. Imme diately within the cutis, which is seen to be to lerably thick, especially at the centre of the dor sum (fig. 747. a.), is seen a dark red stratum, also thick in the last-mentioned situation, thinner at the lateral regions, and again thicker as it curves inwards on each side of the in ferior surface. This is very dense, and cuts with a perfectly even surface. I shall call it the cortical portion (fig. 747.6.). Within this and surrounded by it as by a border, is an area of a more or less oval form, of a paler colour, less dense, and showing a distinctly fibrous character. The fibres appear for the most part transverse (fig. 747. c.), horizontal in the centre, but curved up a little on each side. They are bounded on all sides by the cortical portion. Entering the centre of the bottom of the section, passing vertically up wards, crossing the last-mentioned fibres at right angles, and terminating in the superior surface of the cortical portion, are seen the two genioglossi muscles (fig. 147. d.). These, therefore, constitute a vertical set of fibres ; but they are not the only vertical fibres ; at each side of them, especially at the inferior portion of the section, other vertical fibres are seen passing upwards and a little inwards, and intersecting the more lateral portions of the transverse at right angles.* Cruveilhier has erroneously described them as passing downwards and inwards: their divergence as they pass downwards is very conspicuous. The central area— the lingual nucleus (noyau lingual) of Bauer, — is therefore constituted of two sets of fibres, a ventrical and trans verse ; the transverse being entirely intrinsic, and the vertical in part intrinsic and in part derived from the genioglossus.