Gegenbaur,[42] another well-known authority on comparative anatomy, writes as follows on this subject:—“The hind-gut is longest in the Mammalia, where it forms the large intestine, and is distinguished as such, from the mid-gut, or small intestine. Owing to its greater length, it is arranged in coils, so that the terminal portion only has the straight course taken by the hind-gut of other Vertebrata.”
The two series of facts are not to be disputed. On the one hand mammals are shorter lived than birds and lower vertebrates, on the other hand the large intestine is much longer in them than in any other vertebrates. Is there here any link of causality, binding the two characters, or is it a mere coincidence?
To answer the question we must turn to the function of the large intestine in vertebrates. In the lower members of the group (fish, batrachia, reptiles, birds, etc.), the large intestine is not more than a mere reservoir for the waste matter in the food. It takes no share in digestion, as that is the function of the stomach and the small intestine. Only the cæcum can be thought to have some digestive property. In reptiles, the lowest vertebrates in which the cæcum is present, it is so little differentiated from the large intestine itself, that it is difficult to assign to it any specialised function. In very many birds, however, the cæca are well separated from the main digestive tube. The food material passes into them in considerable quantities, and is retained there sufficiently long for some digestive process to take place. M. Maumus has found, in the cæca of birds, secretions which can dissolve albumen and invert sugar cane, but he has been unable to make out that the cæcal juice has any action upon fatty matter. Such digestive power, however, is slight, and when M. Maumus removed the cæca in fowls and ducks, no evil consequences followed. As in many birds the cæca are rudimentary and in others absent, it may be inferred that these organs are useless, and are in process of degeneration in the class. The cæca can be regarded as playing an important part in the organism only in the case of large running birds, where they are very highly developed, but we have not precise information as to their digestive function.
The variations in the structure in the large intestine are greater in mammals than in birds. In some mammals, the large intestine is a simple prolongation of the small intestine, similar in calibre and in structure. In these conditions it may fulfil a definite digestive function. Th. Eimer[43] has determined that in insectivorous bats the large intestine digests insects like the small intestine. Such cases, however, are rare. In most mammals the large intestine is sharply separated from the small intestine by a valve, and opens directly into the cæcum which may be very large. In the horse, the cæcum is an enormous bag, cylindrical and tapering, generally well filled, and holding on an average 35 litres. It is equally large in many other herbivorous animals, such as the tapir, the elephant, and most rodents. In such cases, the food remains for a considerable time in the organ and without doubt undergoes some digestive changes. In many other mammals, particularly carnivorous forms, the cæcum may be quite absent, whilst in some, as for instance, the cat and dog, it is very small; in the latter cases its digestive function must be non-existent or insignificant.[44]
As for the large intestine itself, apart from the special cases, such as bats, it cannot fulfil any notable digestive function. Th. Eimer was unable to find a proof of any such action in rats and mice, and the very many investigations that have been made in the case of man seem to have established the absence of digestive power in the colon.
Dr. Stragesco,[45] in a recent investigation carried out under the direction of the famous Russian physiologist Pawloff, established that, in normal conditions, digestion and assimilation of food are confined almost exclusively to the small intestine in mammals, and that the large intestine plays only the smallest part. It is only in certain diseases of the digestive tract, in which, on account of increased peristaltic action, the contents of the intestine with the digestive juices are passed quickly from the small intestine to the large intestine, that some digestive work is done in the latter organ.
The large intestine (excluding the cæcum), then, cannot be regarded as an organ of digestion, although absorption of the liquids which have been formed in the small intestine, may take place within its walls. It is known that in the large intestine the contents of the gut give up their water and assume the solid form of fæcal matter. However, whilst the mucous membrane of the large intestine rapidly absorbs water, it has not a similar action on other substances.
The question of the extent to which the large intestine can absorb has been closely investigated, because of its practical importance. It sometimes happens that invalids cannot take food by the mouth, so that their life would be in danger if it were not possible to supply them with food otherwise. Attempts have been made to inject nutritive substances through the skin, or, and this is a more usual procedure, by the rectum. By such means the organism can be kept alive for a certain time, but the absorbing power of the large intestine is extremely small. According to Czerny and Lautschenberger[46] the entire colon of the human being can absorb no more than 6 grammes of albumen in 24 hours, an amount which, from the point of view of nutrition, is very small. It was thought that the large intestine might more rapidly absorb albuminous material which had been previously digested and transformed to peptones, but the experiments of Ewald[47] showed that even in that case the absorption was very small. According to more recent experiments of Heile,[48] carried out upon dogs which had cæcal fistulas, and in the case of a man who had an artificial aperture in the colon, the large intestine does not absorb undigested white of egg, and absorbs water, cane sugar, and glucose only very imperfectly. The only substances which are rapidly absorbed through the wall of the colon are the alkaline fluids from fæcal matter. It is possible, however, to nourish invalids by rectal injections of certain nutritious substances, the most important of which is milk.[49]
The large intestine, which has really very slight digestive properties and cannot absorb any considerable bulk of nutriment, is an organ which secretes mucus. The latter serves to moisten the solid fæcal material, so aiding in its expulsion.
We must conclude, therefore, that the large intestine, the organ so highly developed in mammals, is an apparatus the general function of which is the preparation and elimination of the waste products of digestion. Why should such an organ be so much more developed in mammals than in the other vertebrates?