39. The Comparative Anatomy of the Tongues of the Mammalia. – VI. Summary and Classification of the Tongues of the Primates.

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Summary.

.Pigmentation. – Most of the pigmented forms are included in the Cebidæ. The colour, which is yellow, green, brown or bluish-black, especially the latter, is uniformly distributed all over the dorsum, or the pigmented and colourless parts may form definite patterns; in Ateles grisescens, for example, there is a white cross on a brown background. The vallate and fungiform papilæ may be pigmented (e. g. in Hylobates lar) or colourless, but the lateral organs and central parts of the inferior surface of the tongue are always white. If several examples of each pigmented species are examined it will be seen that the colour varies considerably, so is of no value for purposes of classification. Most specimens of Cercopithecus patas, for example, have reddish-yellow tongues, but some tongues are colourless, and the fungiform papillæ of Cercopithecus tantalus are yellow or colourless.

The bluish-black colour persists longest in preserving fluids.

Form and Proportions. – Most tongues are conical, but a few are oval, spatulate or rectangular; and shape is of no value for comparative purposes.

In most of the Primates the tongue is long and comparatively narrow, but in Anthropopithecus troglodytes and Gorilla gorilla the tongue of the young animal is relatively wider than that of the adult. I did not, however, see such variations in Cercopithecus patas or Macacus sinicus, of which I examined very young and adult examples.

Cunningham showed that the tongue of Simia satyrus resembles that of Homo most closely in the relative proportions of length and width.

The Ape'is round, truncated or pointed, and may or may not have a notch, but the latter is usually absent from the fresh tongue. It is comparatively smooth, or roughened and tuberculated by small conical and fungiform papillæ. The relative quantities of fungiform papillæ on and immediately behind the apex vary; in the Simiidæ and Cercopithecidæ they are numerous and thickly clustered, but in the other families they are few and discrete.

In some species of Lemur the apex exhibits a number of sharp-pointed processes prolonged forwards from ridges on the inferior surface of the tongue, but these vary even in different examples of each species.

Sulci and Ridges. – Few fresh tongues have median dorsal sulci on the oral part of the dorsum, but many preserved specimens- do; and I observed a well-marked median sulcus on the pharyngeal part in Anthropopithecus troglodytes alone. The most pronounced mesial dorsal sulcus which I observed occurred in Mystax ursulus. Median dorsal ridges are present in some Lemuroidea, and Hapalidæ.

Wide, shallow transverse sulci separating low wide transverse ridges are present on the fresh tongues of Lemur catta and Hapcde jacchus; and fine narrow sulci are seen in Gorilla gorilla and Perodicticus potto. Some of the fine sulci and ridges remind one of the patterns of fissures and ridges on the finger-tips.

The median ventral sulcus is present in most tongues, and is never an artefact induced by preserving fluids as is the dorsal one in bottled specimens. It is narrow and deep, or wide and shallow, and it frequently opens posteriorly into a triangular fossa which recieves the upper end of the frenum. In some of the Lemuroidea it receives the median dorsal crest of the sublingua, and it recesses the crest on the dorsal surface of the frenal lamella of Cebus fatuellus.

In Gorilla gorilla, some species of Hylobates and some of the Lemuroidea it contains a fixed crest: and it has been stated that the crest is a remnant of the sublingua, but the presence of both these structures in some Lemurs would seem to disprove that theory.

Lateral Borders. – The edges of the tongue are sharp, or full and rounded, and increase in thickness from before backwards in most species. Those of Chiromys niadagascariensis are more massive in proportion to the size of the tongue than those of any other species of the Primates. Of the Pithecoids Simia satyrus and Anthropopithecus have the most massive lateral borders.

In Tarsius spectrum, Microcebus, Chiromys, and all Lorisidse and Galagidse the lateral borders are devoid of lateral organs and, as I hope to show in a future paper, this has an important bearing on phylogeny. In Gorilla gorilla and Simia satyrus only small parts of the lateral organs are found on the lateral borders, but in all other Primates the greater part is found there.

The conical and fungiform papillæ on the lateral borders are arranged in vertical rows and the points of the former are directed backwards,

Distribution of the Papillœ. – In all Primates except some specimens of Simia satyrus and Symphatamgus syndactylus, some Lemuroidea and Homo, papillæ cover the entire dorsum, apex, lateral borders, and a bounding zone of the inferior surface. In all these species there are smooth non-papillary areas on the base-of the tongue, and in the Lemurs the area is bisected by the median glosso-epiglottic fold.

The ventral papillary zone varies greatly in width, and its characters are of limited value for purposes of classification. It is wide in the Simiidse, Cercopithecidæ, and in Cebus, Ateles, and Lagothrix, but is narrow or absent in all other Primates. In species with a wide zone the conical and fungiform papillæ are numerous, but in those with a narrow zone there are few or no fungiforms. In Gorilla gorilla many of the fine transverse sulci on the dorsum cut the lateral borders and run inwards on the ventral papillary zone.

The Circumvallate Papillœ. – In my paper on the tongues of the Cercopithecidæ I showed that all the papillary patterns present in the family will be seen in most species if sufficient examples of each are examined. The whole series may quickly appear or it may be necessary to examine many. I am not prepared, however, to apply this rule to any other family except the Hapalidæ, as insufficient specimens have come to hand.

In the following list of papillary patterns, P means two papillæ forming a pair, and D.P. indicates four papillæ arranged in a double pair. The Y type means that there are several papillae present and does not include forms in which the four papillæ of a double pair are arranged in a Y.

One can see, therefore, that the Simiidæ and Lemuridæ are the only families whose tongues possess more than four papillæ arranged in a Y, and it will be shown later that they differ from all other Primates in other respects.

The papillæ are round or oval on plan and conical on elevation, with the bases of the cones projecting beyond the Vallums.

The Fungiform Papillœ stretch right across the dorsum, or are absent from the centre thereby forming a dorsal bounding zone. They form a cluster behind the apex, but are arranged in rows of varying degrees of obliquity behind that. The apical cluster is large in the Simiidæ and Cercopithecidæ, but in all other families the transverse rows extend far forwards at its expense. In the majority of tongues with large apical clusters there are many fungiform papillæ on the ventral papillary zone.

It has been shown by Tuckerman that the fungiform papillæ of the apical cluster have many taste-buds.

In some specimens of Anthropopithecus troglodytes there is a row of prominent fungiform papilæ occupying the mid-dorsal line of the tongue and replacing the median dorsal sulcus.

It is sometimes impossible to tell whether a papilla at the posterior part of the oral division of the dorsum is a large fungiform or small vallate form, for fossa and vallum may be indistinguishable even through a strong lens. Histological examination is the only proof. The fungiform papillæ may have no taste-buds or these, if present, lie on the free upper surface of the papilla; in the vallate papillæ, on the other hand, the taste-buds never lie on the free upper surface of the papilla, but are deep down on one or both sides of the fossa.

In some tongues there are more fungiform papillæ than are visible to the naked eye, for some are entirely concealed by overhanging conical papillæ (e.g. Anthropopithecus troglodytes).

The fungiform papillæ on the ventral zone may be thickly clustered at the apex of the tongue and scanty further back, or vies versa, and the examples, and the rows in which they are arranged are close together or discrete.

From the point of view of classification the most important features are the size of the apical dorsal cluster and the presence or absence of the fungiform papillæ on the ventral papillary zone. Although their presence or absence in the centre of the oral part of the dorsum varies greatly, it is not a character of sufficient distinctness to be of value for purposes of classification.

The Conical Papillæ, vary in size and arrangement in the different families, and there are three types of the former: –

1. The papillæ on the pharyngeal part of the tongue are small: – Homo, Simia satyrus.

2. The papillæ on the oral part of the tongue are comparatively small, but those on the pharyngeal part are large and prominent: – Gorilla gorilla, Anthropopithecus troglodytes, all species of Hylobates, all species of Lemur, Chiromys madagascariensis, and Tarsias spectrum. This type also occurs in some lower Mammalia.

3. The papillæ gradually increase in size from the apex of the tongue back to the epiglottis: – Cercopithecidæ, Cebidæ, Hapalidæ, Lorisidæ, and Galagidæ. This arrangement is also present in Microcebus in which the vallate papillæ form a triangle, so the papillæ are of value for distinguishing it from Lemur.

The arrangement of the papillæ distinguishes most of the Cebidæ from all other families. In the latter they form a cluster behind the apex and rows of varying degrees of obliquity behind that, but in the Cebidæ they are dotted irregularly all over the dorsum.

The tongues with smooth non-papillary areas on the pharyngeal part of the dorsum have been enumerated above.

In most Primates the points on the oral part of the dorsum look backwards or backwards and inwards, but in some specimens of Simia satyrus and Cercopithecus œthiops those on the centre of the oral part run in all directions.

In the Cebidæ, Hapalidæ, Lemuroidea, and Tarsioidea the conical papillae are mostly pointed, and cylindrical and globular forms are uncommon. In the other families there is a good admixture of all types.

The Lateral Organs present numerous forms and are of value for purposes of classification: -

1. Organs absent: – Microcebus, Ghiromys, Tarsius, the Lorisidæ and Galagidæ.

2. The laminæ and sulci form ladder-like patterns on the dorsum of the tongue, and only their outer ends cut the lateral borders of the tongue: – Gorilla gorilla and Simia satyrus.

3. The organs are convex towards the lateral vallate papillæ: – Anthropopithecus troglodytes, Hylobates (all species), Lemur (all species).

4. There are rows of short lamina? and sulci on the lateral borders of the tongue: – Cercopithecidæ.

5. The inner borders of the organs are concave towards the lateral vallate papillæ: – Cebidæ.

6. A few faint irregular laminae and sulci are present on the lateral borders of the tongue: – Hapalidæ.

In very few specimens did I find an equal number of laminæ and sulci in the two organs of the same tongue. But one must be careful not to mistake simple folds of the mucosa at either end of the organs for laminæ. Histological examination is the only true test in- doubtful cases, for it reveals the presence of taste-buds in the true laminæ.

In a few species of Cercopithecus one may find fungiform papillæ situated on the laminæ of the lateral organs.

The degree of protrusion of the laminæ and depth of the sulci vary not only in different animals but in several examples of each.

The Lingual Glands are divided into apical and basal parts, but the former is most variable.

The Apical Gland, of Nuhn is present only in Homo and Simia satyrus. Oppel believes that it is a piece which has become cut off from a forward prolongation of the basal glandular mass. The basal mass in the Marsupialia sends forwards two prolongations of variable stoutness, and it is possible that the Apical Gland of Nuhn has been cut off from one of these. If that were so it would support Gegenbaur's view that the tongues of the Primates have evolved from those of the Marsupialia.

The serous and mucous glands on the pharyngeal part of the tongue are developed to an equal degree in the Primates, but the degree of development of the entire basal mass varies.

The pharyngeal part of the tongue possesses a variable degree of development of lymphoid nodules, and a variable number of orifices of pits and the ducts of glands; and the latter are of value for distinguishing the tongues of the various genera of the Cercopithecidæ from one another. Orifices are visible in all Cercopithecidæ, but are abseiit from most Cebidæ, so are of classificatory importance when taken in conjunction with the characters of the lateral organs and mode of arrangement of the conical papillæ. The following are the characters of the orifices in the Cercopithecidæ: –

Genus Presbytes: – Orifices larger and more patulous than in any other genus, and lie in the centres of large round glandular areas. The salivary glands are enormous.

Genus Gercopithecus: – Orifices well-marked, discrete and not patulous, and no white areas present around them.

Genus Macacus: – Orifices like pin points.

Genus Cercocebus: – Small duct orifices present at the sides of the base of the tongue.

Genus Papio: – Large duct orifices present at the sides of the base of the tongue.

In many of the Cebidæ the nodules on the base form zones, with concave anterior borders, in front of the epiglottis, but I did not observe a similar condition in any other family.

The Frenal Lamella varies greatly in the Primates, and appears to belong to the floor of the -mouth rather than to the tongue. It varies even in several examples of the same species, so is of limited value for purposes of classification.

In Homo, Simla satyrus, and some specimens of Symphatomgus syndactylies it appears as two simple folds over Wharton's Ducts; but Mr. Pocock informs me that he observed two well-marked processes in a young Simia satyrus. I cannot believe, however, in conformity with my observations on the tongues of other animals, that reduction in the lamella is a, change due to advancing years. In other species the lamella appears as a triangular or tongue-shaped process with an entire or divided apex and with both edges entire, serrated, or bearing small sharp points. The sharpest and most prominent points on the edges occur in Cercopithecus preussi and some specimens of Cebus fatuellus.

The Orifices of Wharton's Ducts vary in their position on the lamella as follows: –

1. On the upper surface – Gorilla gorilla.

2. On the apex – Anthropopithecus troglodytes.

3. On the under surface – Pithecia satanas.

The apical position is the commonest form, however.

Tuckerman described a rich nerve plexus with peripheral nerve endings in the lamella of Ateles ater. and Gegenbaur considered that the endings were tactile in function. As this condition has not been found in any other tongue there is insufficient material in which to work out its phylogenetic significance.

The Sublingua. – The various Lemuroidea are differentiated from one another by the shape, mobility, and characters of the crests and denticles as follows: –

1. Sublingua triangular or lyrate, has three ventral crests, and is very mobile. The apex is divided into a brushwork of denticles: – Lemur, Hapalemur, Indrisidæ.

2. Sublingua a large flat plate adherent to the under surface of the tongue by its central parts; no denticles present, but a strong keel-like ridge on its under surface projects forwards in the middle line: – Chiromys.

3. Sublingua large, tongue-shaped, but not quite so free as that of Lemur. There is only one median ventral crest, but the dorsal surface has a crest of variable prominence. This crest, the characters of the denticles, and the variations in the consistence of various parts of the sublingua are important: –

In Mierocebus the sublingua is uniformly thick, the median dorsal crest is slight, and the denticles are moderately long.

In Nycticebus and Loris the central parts of the sublingua are thicker than the lateral parts, the median dorsal crest is small and the denticles are of moderate length; they are discrete in the former and close in the latter, but there is no essential difference between the sublinguæ of these genera.

In Perodicticus the median ventral crest is bifurcated posteriorly, the median dorsal crest is very prominent, and the denticles are long and slender.

In Mierocebus, Galago, and Hemigalago the anterior border of the sublingua is broad, but in Loris and Nycticehus it is more or less pointed.

The Plicce Fimbriatœ of the Simiidæ are derived from the sublingua by a process of phylogenetic reduction, and I showed that the plicae of Anthropopithecus troglodytes with the intervening piece of mucosa form a soft triangular field resembling a sublingua; this is even more marked in the tongue of the new-born child, as described and figured by Gegenbaur. The plicæ of Phascolarctos cinereus, however, do not bound such an area.

The tongues of the Gibbons, Cercopithecidæ, Cebidæ, and Hapalidæ illustrate the ultimate stage of reduction, for no traces of the sublingua or plica are present as a rule in the extra-uterine stage. In the foetal Gibbon, as shown by Deuiker, there is a well-marked sublingua; and I observed two minute plicæ in a young Cercopithecus patas, so it is probable that the fœtuses of all Primates have sublinguæ.

If one examines a series of human tongues at different ages, one finds that the new-born child has well-developed plicæ or an actual sublingua provided with taste-buds. As age advances the buds disappear and the plicæ diminish in size. These taste-buds probably account for the more acute sense of taste in the child. Experimental methods also demonstrate that the sense of taste elicited by applications of solutions to the centre of the oral part of the dorsum diminishes as age advances.

This atrophy of structure following loss of function may have played an important part in the reduction in and ultimate loss of the primitive Mammalian tongue.

The Lyita. – Two forms are to be recognised: – The lytta of the tongue and the lytta of the sublingua. And Gegenbaur showed that the latter, when present, appears in one of two forms. In Stenops it forms a strong central supporting rod, but in Tarsius it is double. In Lemur it is absent altogether. Owen described the keel on the ventral surface of the sublingua as the lytta, but sections through the tongue show a well-marked lytta inside; it is connected in the middle to the sublingua.

Ancillary