This is the general plan and rationale of the arrangement, but it is rather an exposition than a description, and it must be understood as merely referring to a transverse vertical section of the tongue, made at the most pos terior part of the free portion of the tongue : there are often irregularities that make it difficult to recognise the plan, and, in some situations, certain disturbing forces, and su peradded parts that quite upset its symmetry. For example, behind the anterior third of the tongue, the genioglossus is seen entering its inferior surface, and displacing all longi tudinal fibres ( fig. 747. d) ; further back, this displacement is more considerable, and we have similar infringements from other muscles; and the intermixture of fat towards the base of the tongue tends materially to upset the regularity of the muscular arrange ment. Yet, in spite of this, it may always be detected, and the average of appearances will be such as I have described.
The muscular fibres are neither straight nor parallel ; those of each system maintain their general direction, but their course is wavy and tortuous, and characterised by the utmost irregularity ; as the fibres pass out wards they branch and sometimes re-unite ( figs. 750, 751.), though their branchings are much more frequent than their re-unions, and hence the fasciculi are smaller and more numerous near the periphery than towards the centre ( fig. 751.) ; by these branchings of the fasciculi each set of fibres, the vertical or transverse, possesses what may be called an intrinsic network, imperfectly marked cer tainly, but sufficient in some parts to mask their parallelism and to break up the rows of longitudinal fibres that are packed between them.
The number of fibres in each of the vertical or transverse fasciculi, varies according to the part of the section viewed, and the situation in the tongue trom which it is taken ; some times one single fibre constitutes the fasci culus, if one may say so, sometimes many dozen. Some of the largest are the most superior of the horizontal,— those that curve up on each side towards the upper surface ( fig. 750.). The same variety of size exists in the discs of longitudinal fibres cut across; the number of fibres in them may be counted, from two or three to thirty or forty : those nearest the surface are certainly the smallest, and they do not completely fill the meshes of the muscular network through which they pass (fig. 751.), but a certain quantity of fibrous tissue dips down among them : this, however, only for a little way. The shape of the longi
tudinal bundles is as various as their size— circular, polygonal, triangular, elliptical, in fact, every conceivable shape (fig. 751.); they seem moulded by the fibres among which they lie, or, more correctly, they and the others among which they lie, mutually regu late each other's shape and direction.
The peculiarity, then, of the arrangement of the intrinsic muscles of the tongue is this :— that there are three sets of fibres passing through the same area, and acting in three different directions ; that these three direc tions are, in the main, at right angles the one to the other, in fact, that they coincide with the three axes of the cube ; that to facilitate this arrangement, a beautiful system of pack age is adopted, whereby each of these is enabled to pass in a straight line to its destin ation without being interfered with by the other two ; whereby the individual bundles of each set are isolated from their fellows ; whereby the whole of them are contained in the smallest possible compass ; whereby they not only admit the passage of, but mutually support and conduct each other ; whereby, in consequence of this, they are enabled to dis pense with the support of cellular tissue, which accordingly we find absent ; whereby, lastly, the tongue contains the greatest amount of muscular tissue possible for its bulk. This system of package consists of this — that the crossing of the fibres of any two sets forms a lattice work, or mesh, through which the third shall pass, and that the successive layers of the crossing fibres shall be so arranged that the areolm shall form continuous channels for the transmission of the perforating ones. In whatever plane we look at the fibres, we find that this is the case—that two sets are crossing fibres and one set perforating — that two are seen in profile, one in section ; but, as we vary the plane, so do we vary the ap pearance of the fibres, one set alone remain ing the same and two interchanging. Thus, in a transverse vertical section the transverse and vertical fibres are seen in profile, and the longitudinal in section ; in a longitudinal ver tical section the vertical and longitudinal are in profile, and the transverse in section ; while again, in the horizontal section, the trans verse and longitudinal are seen in profile, and the vertical in section. So, no set can be called a perforating set or a crossing set, they arc all equally so.