Cæcum

Cæcum (Lat. cæcus, 'blind'), a general term for any blind sac arising from the gut. Such outgrowths are common in many different classes of animals. The gut being a tube lined by cells in very favourable nutritive conditions, and being at the same time limited in its growth at both ends, it is easy to understand why it should develop outgrowths, or become twisted upon itself in the way it does. Insects and starfishes afford good illustrations of cæca attached to the gut of invertebrate animals. Among backboned animals cæca occur on all the three divisions of the gut. (a) The crop of birds is a more or less marked cæcum or outgrowth of the fore-gut. The stomach itself is frequently, as in many fishes, of the nature of a cæcal sac from the general course of the fore-gut. The complex stomach of ruminants, &c. illustrates the same sort of formation. (b) The digestive cæca at the beginning of the small intestine in many fishes are good examples of outgrowths from the mid-gut. (c) At the union of small and large intestine (hind-gut), cæca are often given off in reptiles, birds, and mammals. Those of birds are usually two in number, and vary greatly from mere papillæ to long tubes. The cæcum of mammals is usually single. It varies greatly with the diet, being short for instance in carnivora, absent altogether in bears and weasels, but long in herbivorous animals. As a hollow sac continuous with the rest of the intestine, it delays the passage of the food, which is thus subjected to longer digestion and absorption. In man the cæcum terminates in a small appendage which has not grown in proportion with the rest, and is well known as the appendix vermiculiformis. Besides the above structures to which the term is often applied (especially to those of the hind-gut), it may be noted that the lungs are outgrowths from the fore-gut, the liver a paired outgrowth from the mid-gut, the allantois an outgrowth from the hind-gut. In their primitive embryonic stages, and in their first appearance—e.g. as air-bladder of fish, hepatic sac of lancelet, bladder of frog—these may be fairly described as cæca. For other information, and for the human cæcum, see DIGESTION, and the other articles named at BOWELS.