In case of gravel, also, where the concrements consist of uric acid, the administering of such fruit can give beneficial results. Besides mineral salts and vegetable acids, fruits and vegetables contain a third important substance, which is cellulose, the framework of their cell tissues. Although this is very difficult to digest, still there can be no doubt, from the experiments made on animals and also on man by Weiske,[272] that cellulose is also a nourishing food, for he proved on himself and another person that from 46 per cent. to 65 per cent. of the cellulose can be digested. Footnote 272: Weiske: Zeitschrift für Biologie, vol. vi, p. 456. The chief advantage, however, of cellulose does not lie in its nourishing properties, which are not great, but in the fact that it acts as the best natural stimulus to the peristaltic movements of the intestines. Thus food that contains such a residue (which is contained most largely in vegetables and fruit) is also the best to use if we desire to keep the intestines open and to observe the most important precept of their hygiene. This hygienic condition can also be much advanced by vegetables of the cereal kind, which, as shown in the chapter on hygiene of the intestines, may act as a disinfectant of the same through the milk acid that is formed therefrom in the intestines. Vegetables and fruit have thus very great advantages, and even in winter our daily diet should consist plentifully of them, as grape-fruit, oranges, etc., can be obtained at that time of the year. But when vegetables and fruit are exclusively used as a diet they present certain dangers, as we point out in the next chapter on the advantages and disadvantages of a vegetarian diet. ------------------------------------------------------------------------ CHAPTER XXXVII. ON THE ADVANTAGES AND DISADVANTAGES OF A VEGETARIAN DIET. OWING to certain peculiarities in our anatomical construction we are not intended by Nature to be vegetarians. This is amply demonstrated when we consider the formation of our teeth. These are neither the teeth of carnivorous nor of herbivorous animals. We have, in fact, teeth similar to those found among omnivorous animals, such as the dog and pig, while our whole metabolism, the transformation and assimilation of food in our bodies, presents great similarity to that of the dog.