The small intestine is the segment of the alimentary canal in which digestion of the non-nitrogenous food substances takes place, and which affords the necessary area of mucous surface for the absorption of all digested matters. Consequently the character and habitual quantity of the food here elaborated exerts a very marked influence on the _length_ of the small intestine, _i. e._, on the extent of the digestive and absorbing surface represented by its mucous membrane. The relative length of the small intestine in any individual form will vary with both the quantity and volume of the food and with the rapidity of the metabolic processes. Animals, in which digestion is rapid and the usual food small in bulk and concentrated in its nutrient qualities, have a relatively short intestine, while the canal is longer in forms subsisting on food which is bulky and which demands considerable time for its elaboration. Hence we find the relatively shortest intestine in carnivora, the longest in herbivora, while the canal in omnivora occupies an intermediate position in regard to its relative length. The rapidity of tissue-metabolism also exerts a marked influence on the length and development of this portion of the alimentary canal. In the warm-blooded animals (mammals and birds) the tissue-changes are constant and rapid and call for a large amount of nutrition within a given period, while the metabolic processes in the cold-blooded vertebrates (reptiles, amphibia and fishes) are slow, these animals being able to go without food for long periods. Consequently in the former class the small intestine is relatively much longer than in the latter. Thus in certain birds and herbivorous mammals the small intestine exceeds the total length of the body many times. This influence of the quantity and quality of the food on the length of the intestinal canal is seen, for example, very well during the course of development in the frog.