Moreover, the movements affecting the length of the tongue may concur with those affecting its direction; for instance, on apply ing the tip of the tongue to the root of a canine tooth of the upper jaw, on the outer surface of the alveolus, the tongue is elon gated, and vertico-laterally flexed; on apply ing it to the last molar tooth of the lower jaw it is laterally flexed and shortened.
Secondly, the extrinsic movements of the tongue admit of the same division as the intrinsic, into those regulating its length, and those regulating its direction or shape.
a. Thus the tongue is carried upwards and backwards by the styloglossus, assisted by the other styloid muscles, downwards and backwards by the hyoglossus, directly back wards when these both concur ; and it is carried forwards and protruded from the mouth by the genioglossus. These move ments, en masse, almost always concur with the intrinsic movements, the whole organ following to a certain extent the direction of the extremity.
b. But the extrinsic muscles affect very materially the direction and shape of the tongue : the styloglossi raise and expand the sides, the palatoglossi raise and approximate them, and the hyoglossi depress them ; the one set makes the dorsum of the tongue transversely concave, the other convex : moreover, the posterior fibres of the genio glossus draw the centre of the tongue for wards and downwards, so that they also render the tongue transversely concave. The action of the genioglossus is peculiar ; the most posterior fibres draw forwards the base of the tongue, and are those chiefly concerned in the protrusion of the organ ; the most anterior concur in replacing the tongue when thus protruded ; when the whole muscle acts it compresses the tongue into a sort of button, and carries it deep down in the arch of the lower jaw. The relations of the hyogiossus and genioglossus are worthy of remark : they are congeners, inasmuch as they both tend to draw the tongue downwards ; they are antagonists, inasmuch as the one tends to draw the tongue principally forwards, the other principally backwards ; when all four muscles act, the tongue is depressed deep in the jaw, but further back than when the genioglossi act alone ; when the two muscles of one side act, that side alone is depressed, and a certain torsion is given to the tongue which enables it to apply the tip advantageously to parts that it would otherwise be very difficult to reach.
If the movement of the tongue in transferring the tip from the last inferior molar tooth of one side to that of the other be watched, with the mouth open, at a glass, it will be seen to be collected in a globe at the back of the mouth, and to rotate horizontally, as it were on a pivot. The only way in which I can conceive this movement to be brought about, is by the consentaneous action of the hyo glossus of the one side with the genioglossus of the other,—the one right with the other left, then the one left with the other right, and so on, the styloglossi and palatoglossi at the same time preventing the depression of the tongue.
But it would be vain to attempt to describe in words the endless variety of movements of which the tongue is susceptible ; and if it were possible to give an idea of them, space could not be afforded: so, dismissing this part of my subject, I will proceed to the next.
3. Tegumentary system.—The tegumentary system of the tongue is formed by the mucous membrane of the mouth passing on to it from neighbouring parts, and undergoing special modifications according to the part that it invests. A superficial glance shows it at once to admit of a triple division, into, first, a really or apparently plane portion, situated in front of the epiglottis, beneath the borders, and on the free portion of the under surface ; secondly, a papillary portion, covering the anterior two-thirds of the upper surface, the free borders, and the tip ; and, thirdly, a glandular portion, occupying the posterior third of the upper surface, where it is folded into little crypts and raised in nodules over small mucous glands : these glands exist also along the sides and beneath the tip : they will be reserved for future description.
The mucous membrane here, as elsewhere, consists of three portions,—a basement or limitary membrane, underlaid by a submucous areolar tissue, and surmounted by an epithe lium.