The genioglossus, the largest of all the mus cles of the tongue, which it connects to the lower jaw, is of a radiated or fan-shape, and is placed vertically in immediate contact with its fellow of the opposite side. It arises from the superior genial tubercle of the lower jaw by a tendinous tuft from which the muscular fibres radiate to their different destinations. The most anterior, the shortest, forming the anterior free margin of the muscle, pass up wards and forwards to the tongue, having reached the under surface of which, they are continued, according to Cruveilhier, on that surface to the tip ; but in all the specimens that I have examined they appeared to con tinue an oblique course to the dorsum. The succeeding fibres pass more and more back wards, and having reached the inferior surface of the tongue, are directed vertically to the dorsum, into the middle line of the whole of which they are inserted, from apex to base. The two muscles may be separated up to the point of their immergence into the tongue, but beyond that line their separation is no longer possible ; for, having entered the tongue, they come into relation with the transverse intrinsic fibres, which they cut at right angles, and interlacing with which they pass to the cutis of the dorsum, forming part of the vertical intrinsic muscle, from the rest of which they are not to be dis tinguished except by their mesial situation. The fibres do not curve outwards, as sup posed by Alarjolin, to form part of the transverse lingual; nor do they expand at all at their insertion, as stated by Cru veilhier ; there is no disposition to lateral divergence in any part of their course ; on the contrary, their direction is rather upwards and inwards throughout, and their insertion extremely narrow — a mere line —as it might be imagined it would be, when its great longitudinal extent is remembered. The longitudinal furrow is, I think, mainly produced by the traction of this muscle in the median line. The most inferior fibres pass backwards and downwards, and are inserted into the hyoglossal ligament. Some of the fibres immediately above pass backwards, ac cording to some authors, to the sides of the pharynx, where, uniting with the middle con strictor, they form the genio-pharyngiens of Winslow. Relations: Internally with its fellow, being separated merely by areolar tissue con taining some fat, particularly at its inferior part and the median fibrous lamina, when the in termuscular areolar tissue is sufficiently dense to deserve that name ; anteriorly, with the frx num, to which it is subjacent; inferiorly, with the geniohyoid; and externally, with the sub lingual gland, the mylohyoid, hyoglossus, and inferior lingualis muscles, the ranine artery, and the gustatory nerve. The hypoglossal nerve threads its fibres, and passes forwards among them. Action : To raise and draw forwards the tongue ; to assist in constricting the pharynx ; to protrude, retract, or depress the tongue in the mouth, according as all or part of the fibres are employed ; to depress the centre of the tongue, and render it concave, from side to side.
c. Accessory extrinsic muscles. — These are, in short, all the muscles that move the os hyoides without being attached to the tongue, for whatever moves the hyoid bone must move the tongue, which is fixed to it. They are accessory to the proper extrinsic muscles in two ways, either by acting in con cert with them, or by facilitating their action on the tongue by rendering the hyoid bone a fixed point. Thus, in the first method, the stylohyoid and posterior belly of the digas tricus concur with the styloglossus in draw ing the tongue upwards and backwards. In
the same way, the anterior belly of the digas tricus, the mylohyoid, and geniohyoid, con cur with the inferior portion of the genio glossus in raising and drawing forwards the hyoid bone, and facilitating the protrusion of the tongue from the mouth. By the second method the muscles from the styloid process to the hyoid bone assist the longitudinal in trinsic muscles of the tongue by rendering the base a fixed point from which they can ad vantageously act on its length, and in the same way the inferior set of hyoidean muscles are accessory to the hyoglossus by fixing the hyoid bone down. More might be said on this subject, but enough has been stated to indicate the important relation of these muscles to the proper muscles of the tongue, and for the sake of brevity that may suffice.
Movements of the the infinite variety of movements by which the tongue is, by virtue of its complex muscular organisa tion, susceptible, may be arranged under two heads, —its extrinsic and intrinsic move ments ; sometimes dissociated, more fre quently concurrent: for the sake of clearness I shall consider them separately, and then group them.
First, the intrinsic movements of the tongue are of two sorts ; those affecting its length, and those affecting its direction.
a. As affecting its length. The elongation of the tongue is provided for, like all in trinsic elongation, by diminution of calibre ; in the tongue this is produced by transverse and vertical contraction, especially the transverse, whereby the tongue becomes at once elon gated and pointed ; by this means the tip of the tongue can be protruded beyond the teeth without any movement of the organ en masse, or any assistance of the extrinsic muscles. From this elongated state it is restored to its original position and shape by the contraction of all the longitudinal fibres composing the cortical portion, which draw it directly back, the transverse and vertical fibres at the same time ceasing to act. Thus we see that the central and cortical portions of the intrinsic muscles are antagonistic ; but they are capable of association : for instance, when the tongue is to be flattened and its sides pressed against the teeth without any elongation, an action very frequent in mastication and in the pro nunciation of some letters, this is done by the contraction of the vertical fibres associ ated with the longitudinal, the one diminishing the vertical thickness, and so spreading the tongue out, the other preventing the elonga tion which the diminution of vertical thick ness would otherwise be attended with.
b. As affecting its direction. The direction of the tongue is entirely regulated, as far as the intrinsic muscles go, by the longitudinal fibres, and their power of modifying the direc tion of the tongue, as well as shortening it, depends on their power of partial action : thus the lateral lingual of one side can act, or of the other ; the superior .or the inferior, and the point of the tongue is of course moved to the side of the acting muscle. This modi fication of the direction of the tongue is, per haps, the most complete movement that it possesses; it is certainly the most extensive : by it the tip of the tongue may be depressed deep below the incisor teeth, or reflected back on the soft palate, so as nearly to touch the uvula; or laterally, from the pillars of the fauces on one side it may be carried round the cheeks and alveolar arches to the same position on the other side : these are the cardinal points, up and down, right and left; they may be united in any proportions, so as to carry the extremity of the tongue to any intermediate position.