Smooth muscle has the duty of operating the internal organs. For this function no great strength is required; the motions do not have to be powerful. Nor is rapid motion important. Smooth muscle does not have to be so highly developed, then, as is skeletal. It is sluggish and rather feeble in its actions. There are, however, two points of superiority about smooth muscle, which fit in well with its special task. The first of these is its freedom from fatigue. There are in the body numerous smooth muscle masses that are in contraction practically all the time. This would be impossible if fatigue were to develop. These masses make up what are called the _sphincters_, rings of muscle surrounding openings like that from the esophagus to the stomach or from the stomach to the small intestine. It is the duty of these sphincters to hold the openings closed all the time except occasionally when they open for just an instant to let material through. The second point about smooth muscle which fits it for its work is that it is capable of stretching out greatly or contracting sharply without much difference in the force with which it is pulling. For example, at the beginning of a meal the walls of the stomach are drawn up, so that the food that is swallowed enters a small space. With the progress of the meal the stomach enlarges, so that at the end it has a much greater bulk than at the beginning. But the actual pressure of the stomach upon its contents is about the same as at the beginning. If the stomach were an ordinary elastic bag this could not happen; the walls would have to stretch as the stomach filled, and the stretching would mean greater pressure. Since the stomach walls are of smooth muscle they adjust themselves to the progress of the meal. It is important to note that there is a limit to this possibility of adjustment. If one is so greedy as to keep on stuffing after the stomach has reached its full size, stretching does occur, and if this is repeated it may lead to a diseased condition known as “dilated stomach,” which will cause much digestive trouble. CHAPTER VIII SENSATION--INTERNAL AND CONTACT SENSES