We have so far described the alimentary canal as a single layer of cells, but it must be obvious that these soft secreting portions of the tube are not capable of vigorous movement. The canal proper is surrounded by a tough sheath of connective tissue which prevents its being overdistended or ruptured, and, by means of a layer—or, rather, two layers—of non-striped muscle which it contains, produces the movements which result in the passage of its contents along the tube. These two layers lie well to the outside of the connective-tissue sheath. The fibres of the inner layer are arranged circularly, so as to form rings round the tube; those of the outer have a longitudinal direction, running, therefore, parallel with its long axis. When the former contract, the diameter of the tube is reduced, while contraction of the latter has the effect of enlarging it. (See Diagram 32.) The movements of the intestine are what is known as peristaltic. Contraction of the muscle fibres is not simultaneous in all parts, but passes in waves along it. Just in front of the food the longitudinal fibres contract, and thus offer less resistance, while just behind the circular fibres reduce the size of the tube, and so get up a pressure. The result of a number of successive waves of contraction passing down the alimentary canal is that the food is propelled along it. [Illustration: DIAGRAM 32.—TO ILLUSTRATE THE PASSAGE OF FOOD ALONG THE INTESTINE.] The arrangement of the muscle varies in places to suit special needs. Where the tube suddenly enlarges to form the stomach, and where the stomach suddenly narrows to the intestine, there are two strong rings of muscle, whose constricting influence converts the enlargement into a closed chamber during gastric digestion; while the coats which actually clothe it here run obliquely, and their activity causes the contents to be slowly churned about inside. Thus it will be seen that it is not only the voluntary muscles which give the alimentary system its opportunities; without these unobtrusive non-striped cells we should toil for our bread and swallow it in vain. V. Our next step, after having surveyed the principle of movement by which the chemical necessities of the body are exposed to its absorbing surface, must be to see how the fluid which transports them is made to pass along the tubes containing it. We have already had occasion to describe how these blood and lymph vessels ramify through all the organs, when we were dealing with the chemical influence of the blood and lymph. The tubes through which the lymph is brought back to the blood-stream have thin walls, and no muscle of their own. They are subjected, however, to a constantly varying pressure by the movements of the limbs and trunk, and as, owing to valves inside them, the lymph can only escape in one direction, there is a constant flow towards the junction with the bloodvessels.