The digestive system may be conceived in its simplest form as a muscular tube into which glands all along its course pour secretions. Some of these glands are embedded in the wall of the tube ; some, such as the pancreas and the liver, are so large that they lie outside and discharge their secretion through a duct which empties into the digestive canal. The muscular action of the walls of the canal pushes the food ever onward.
It must always be remembered that, though for purposes of analysis we shall study them separately, these dual functions of movement by the muscular walls of the digestive canal and of chemical action by the digestive juices go on simultaneously and are interdependent. The churning of the stomach and intestines mixes the food and breaks it up so that the juices can get to every part of it, then the onward movement carries the residue and waste away so that they can be evacuated.
The movements in the mouth consist of the grasping or prehensile movements of the lips, the grinding action of the teeth, and the rolling of the bolus of food by the tongue and cheeks so that it is intermixed with the salivary secretion which is being poured out. These movements and the act of swallowing are voluntary; after they are completed, the movements of the food are beyond the control of the will.
The stomach is a bag the walls of which are largely made up of involuntary or smooth muscle-fibres. It is marked off from the intestines by a strong circular muscular band, the pylorus. During digestion in the stomach the pylorus remains closed, except at intervals when it opens momentarily to allow the ejection of a well-digested bolus of food into the intestine. The movements of the stomach, technically called peristalsis, consist first of a series of waves of contraction running from the upper end towards the pylorus.
There is another stomach movement, a turning or churning, a rolling from side to side like an electric washer. Both these movements, the contraction wave and the rolling, are seen daily by the physician with the X-ray. They are controlled by the automatic or vegetative nervous system, which sends two sets of fibres to the stomach – one set accelerating the movements and one set inhibiting them. These nerve-fibres are co-ordinated in one of the large ganglia of the vegetative nervous system – the solar plexus – which, owing to the discomfiture of one Mr. Sharkey, as I remember it, was the first anatomical term I ever learned.