The digestive canal has, however, another function. The cells which compose it have not only to secrete juices, to convert the food into a usable form; they have then to absorb it. The nearer a particle of food is to the wall of cells, the sooner it is reached by these juices, and the less chance there is of useful material being swept away and lost. In view of this fact, along certain tracts the digestive canal is folded inwards, and there are projections, which increase the number of cells to secrete and their opportunities of absorption. (See Diagram 8.) [Illustration: DIAGRAM 9.—SHOWING HOW GLANDS ARISE.] Here again we have an illustration of a constantly recurring need, with a device for meeting it—increase of surface without increase of bulk. We met with it before in the cellular system; we shall meet with it again in glands, lungs, and brain, at least. The importance of a device for gaining this end is apparent when one remembers what the comparative value of surface and bulk is to an animal, and that, while surface increases by the square, bulk increases by the cube. The principle is pressed to an extreme, together with the allied principle of division of labour, in glands. The object of these is to increase the number of secreting cells, and, as they are delicate, to keep them protected from contact with coarse particles of food. And, in order that nothing may interfere with their efficiency, they are absolved from the duty of absorbing. Hence tubes grow out from the cavity of the alimentary canal lined with the same cells, but, as no food ever enters, the cells which line them devote themselves entirely to pouring out digestive juices. Glands differ considerably in structure and in the liquids which they secrete. Some are very small; some, like the liver, very large. In some the tube is very short, in some long, coiled and branched, and sometimes the gland is connected with the surface by more or less of a duct. Some glands only secrete one enzyme, some several. In each, however, the principle is that shown in Diagram 9, no matter how its structure is masked by the bloodvessels and supporting cells or connective tissue which envelop it.