The digestive system of anthropoids likewise affords interesting points of comparison. The cavity of the mouth is, as we have seen, bordered by large and flexible lips. The mucous membrane of the mouth and the gums are flesh-coloured; they assume a darker colour in older animals, and are then sometimes marked with spots of a bluish or brownish grey. Ehlers describes, as a peculiarity in the mucous membrane of the mouths of the gorilla and chimpanzee, that there are what he calls buccal folds, which pass on both sides from the fore surface of the upper and lower jaw into the mucous membrane of the cheeks, and are of the height of the canine teeth.[75] I have myself only observed these folds in the gorilla, of which an illustration is given in Fig. 3, and not in any other specimen. I have observed scarcely any indications of these folds in other anthropoids, and then only of such a doubtful nature that I am not disposed to regard the circumstance as of any special significance. A small band on the upper and lower lips, sometimes only slightly developed, but always perceptible, is present in all anthropoids. The tongue is small, and not provided at its base with several great concave follicles as in man; these are at least only faintly represented, and not easy to observe. Around them there rise pock-like, tufted warts, very close together, which in an aged gorilla are apt to become hard and horny. These are also prominent between the follicles of the tonsils. The circumvallate papillæ of the tongue are less numerous than in man, and often, especially in the chimpanzee, they take the form of a cross, or of the letter T, or in the gorilla of a V. The uvula and palate present no special variation from the human type. On the hard palate there are a number of folds, or rather swellings, which extend laterally from the central suture of the palate, towards the row of teeth in the upper jaw; these are sometimes simple, sometimes complex, and vary in their details in individual cases. They are particularly marked in the adult chimpanzee, and are also very apparent in the gibbon, and they are arranged with a somewhat ornamental regularity. These inequalities are not altogether insignificant in the human palate, but this subject has not been much studied since Gegenbaur directed the attention of scientific men to them, and special light has been thrown upon it by Bischoff and Ehlers, as far as anthropoids are concerned.