Though the functional changes which have taken place in the digestive system of the evolving man have necessarily a structural basis, those changes are, as far as they can be detected, far less marked than might perhaps be expected. There is a notable general resemblance between the digestive organs of man and of the great apes, a fact which shows that these organs in man have undergone very little visible change since simian times. One of their most striking anatomical differences is in the valvulae conniventes, which in the apes are rudimentary only. Another pronounced difference is the greater muscularity of the simian stomach, a character which fits it to cope with the coarse varieties of vegetable tissue which enter so largely into the food of the apes.
We are, unfortunately, not able to state minutely in what respects the digestive organs of existing pre-cibiculturists differ anatomically from those of the neo-man. It is probable that the digestive tube is larger and more muscular in the former case.
The dental formula of the apes is the same as for man, though the teeth of the former are larger and stronger than those of the latter, and therefore adapted for coarser and more vigorous work. Since man, before he learnt to break up the cellulose framework of his vegetable food by cooking, grinding, and other means, was compelled to make vigorous use of his maxillary apparatus, we may be sure that in the pre-cookery period his jaws and teeth were correspondingly strong and massive, but when, with the discovery of artificial means of disintegrating the cellulose, mastication was in great measure relieved of one of its chief functions, they began to get smaller, while dental caries, hitherto almost unknown, became less rare, invading chiefly the third molars ("wisdom" teeth). Again, as the effect of agriculture was to reduce the cellulose constituents of vegetable food and thus to render it more easy of mastication, we find the jaws and teeth further diminishing in size during the agricultural period and diseases of the teeth increasing in frequency. These effects were not, however, pronounced during the early agricultural epoch, partly because man still continued to eat freely of raw vegetable food, and partly also because much of his cooked vegetable food needed, owing to its coarseness, considerable mastication. It is not until we arrive at the neo-agricultural stage that the effect upon jaws and teeth of food artificially produced and prepared becomes pronounced. In consequence of the softness of modern food the jaw does not grow to its normal size, with the result that the teeth, whose growth is not affected to the same extent, are often unable to take up their normal positions. For these and other reasons associated with the softness of the modern vegetable dietary dental diseases have assumed alarming proportions.