The mouth is closed and the voluntary muscles of the tongue and cheeks push the bolus backwards into the pharynx.
The
muscles of the pharynx are stimulated by a reflex action initiated in
the walls of the oropharynx and 
coordinated
in the medulla and lower pons in the brain stem. Involuntary
contraction of these muscles propels the bolus down into the
oesophagus. All other routes that the bolus could take are closed.
The soft palate rises up and closes off the nasopharynx; the tongue
and the pharyngeal folds block the way back into the mouth; and the
larynx is lifted up and forward so that its opening is occluded by
the overhanging epiglottis preventing entry into the airway
(trachea). 3. The presence of the bolus in the pharynx stimulates a
wave of peristalsis that propels the bolus through the oesophagus to
the stomach.
Peristaltic waves pass along the oesophagus only after swallowing begins (see Fig. 12.4). Otherwise the walls are relaxed. Ahead of a peristaltic wave, the cardiac sphincter guarding the entrance to the stomach relaxes to allow the descending bolus to pass into the stomach. Usually constriction of the cardiac sphincter prevents reflux of gastric acid into the oesophagus. Other factors preventing gastric reflux include:
the attachment of the stomach to the diaphragm by the peritoneum
the maintenance of an acute angle between the oesophagus and the fundus of the stomach, i.e. an acute cardio-oesophageal angle
increased tone of the cardiac sphincter when intraabdominal pressure is increased and the pinching effect of diaphragm muscle fibres.
The walls of the oesophagus are lubricated by mucus which assists the passage of the bolus during the peristaltic contraction of the muscular wall.
Stomach
O
rgans
associated with the stomach
(Fig. 12.19)
Anteriorly - left lobe of liver and anterior abdominal wall
Posteriorly - abdominal aorta, pancreas, spleen, left kidney and adrenal gland
Superiorly - diaphragm, oesophagus and left lobe of liver
Inferiorly - transverse colon and small intestine
To the left - diaphragm and spleen
To the right - liver and duodenum.
Structure of the stomach (Fig. 12.20)
The stomach is continuous with the oesophagus at the cardiac sphincter and with the duodenum at the pyloric sphincter. It has two curvatures. The lesser curvature is short, lies on the posterior surface of the stomach and is the downward continuation of the posterior wall of the oesophagus. Just before the pyloric sphincter it curves upwards to complete the J shape. Where the oesophagus joins the stomach the anterior region angles acutely upwards, curves downwards forming the greater curvature and then slightly upwards towards the pyloric sphincter.
The stomach is divided into three regions: the fundus, the body and the antrum. At the distal end of the pyloric antrum is the pyloric sphincter, guarding the opening between the stomach and the duodenum. When the stomach is inactive the pyloric sphincter is relaxed and open, and when the stomach contains food the sphincter is closed.
W alls of the stomach
The four layers of tissue that comprise the basic structure of the alimentary canal (Fig. 12.2) are found in the stomach but with some modifications.
Muscle layer (Fig. 12.21). This consists of three layers of smooth muscle fibres:
an outer layer of longitudinal fibres
a middle layer of circular fibres
an inner layer of oblique fibres.