Again, we see that the office of the longi tudinal fibres requires that they should have that special superficial arrangement which is the only one left them by the necessary dis position of the other two. The chief office of the longitudinal fibres is to alter the direction of the tongue longitudinally, to twist it from side to side, or up and down ; any thing but a superficial distribution would render them powerless for this act. For if the longitu dinal fibres were placed in the centre, it is evident that they could only shorten the tongue; but, being arranged superficially, when a portion of them contracts, that side of the tongue on which the contracting ones are is shortened more than the rest ; in other words, the tongue is turned towards that side ; and it is only when the whole sheath of longi tudinal fibres acts equally that the tongue is contracted directly backwards.
Having premised this general description, I shall now proceed to give a particular ac count of the microscopical appearances of successive sections made in the three prin cipal planes—the transverse, the longitudinal, and the horizontal. I shall begin with the transverse as being the clearest and the most illustrative.
The first transverse vertical sections, made at the tip of the tongue, of course remove successive portions of the papillary structure : we next come to the cutis—the dense areolar tissue subtending the papillae — and many sections are made before the appearance of any muscle ; we have in fact to get through the thickness of the cutis. The first muscular fibres that make their appearance are the transverse, consisting of a single slender bundle of fibres in that direction, occupying nearly a middle plane between the upper and under surfaces, lying horizontally, collected into a single bundle in the centre, but breaking up at each end into smaller fasciculi, which diverge as they pass to their insertion into the cutis at the sides of the tongue, so as to gain a more extended attachment. The sections following this display an increasing quantity of this muscle, the diameter of the unbroken bundle in the centre being greater, and the fasciculi into which it divides at the sides more numerous ; but as yet no other system of fibres has appeared. The next addition, as we proceed backwards, is that of vertical fibres, which are at first very few and scanty, and placed not in the centre, but in two sets, one on each side of the centre ; they converge a little as they pass upwards, and are rather curved, presenting their concavity outwards. The succeeding sections
show them increasing in numbers and spread ing towards the centre, where they finally meet, and then the central part of the tongue is con stituted in the same way as it is throughout the whole succeeding length of the organ, namely, by decussating vertical and transverse fibres ; but as yet no longitudinal fibres have appeared, and they are not seen till after the transverse area of the tongue has been entirely occupied by the vertical and hori zontal fibres as above described. They first appear at the inferior surface, then at the sublateral and lateral regions ; next they are seen at the centre of the upper surface in a small definite cluster, from which they spread out, and so complete the circumference of the tongue.
We see from this that the fibres that occupy the extreme point of the tongue are the transverse; that the next met with are the vertical ; and that the longitudinal do not extend so far forward as either of the other two. This is what might be expected. The chief muscular; requisition at the extremity of the tongue is the power of pointing it : the shape of a tactile part is eminently subservient to its power of touch, because exact localisation, which is a most impor tant element in touch (if it is not the very essence of it), depends on the smallness of the touching part. Now the extremity of the tongue, of its ordinary broad, flat shape when at rest, would be a very poor tactile instrument, and very far removed from a form that would capacitate it for the minute ap preciation of distance and form. In what way, then, is the desired pointed shape of the tip of the tongue to be produced ? Manifestly by transverse contraction, for it is by the spread of the tongue in this direction that it departs from the pointed form ; and so we see the transverse fibres continued beyond either of the others, and occupying at the extreme tip the whole of the space assigned to the muscular structure. Furthermore, the longi tudinal fibres are not necessary at the extreme point, either to flatten or shorten the tip, which is brought about by its own elasticity immediately on the cessation of the con traction of the transverse fibres, or to move it in any direction, which is done rather by the movements of the parts coming immediately next the tip than of the tip itself. Thus, both negatively and positively, we see a reason for the continuation of the transverse fibres further forwards than either of the other two sets.