THIRD STAGE OF DEGLUTITION. 505 



presents no cavity, but, in its centre, a radiating or branching cleft, 

 formed by the meeting of the plicated folds. The pharynx is per- 

 manently open, as far as the aperture leading into the larynx, but its 

 lower portion, and the whole length of the oesophagus, are habitually 

 closed, their sides being always in contact, excepting when solids, 

 fluids, or gases are passing through them ; they are examples of what 

 are called potential cavities. When, however, any solid or fluid is 

 passing down the oesophagus, the longitudinal plicae of its mucous coat 

 are obliterated. This membrane is beset with papillae, and covered 

 with a many-layered squamous epithelium, which, at the lower end of 

 the oesophagus, at the line of junction with the stomach, abruptly 

 changes its character, and presents a crenulated border. The mucous 

 membrane of the oesophagus is provided, especially at its upper and 

 lower ends, with small compound mucous glands. 



In the third and final stage of deglutition, the food, pressed down 

 by the muscles of the pharynx, first distends the walls of the oesopha- 

 gus, the muscular coat of which, however, speedily contracts above 

 the morsel, and so urges it further downwards; the part thus dilated, 

 then contracts above the mass of food, which is thus driven on, and so, 

 by a succession of similar acts, is propelled, in separate portions, into 

 the stomach. This successive contraction of the muscular coat of the 

 oesophagus, from above downwards, is called vermicular or peristaltic. 

 The circular fibres contract, in a wave-like manner, from above down- 

 wards, and are the propulsive agents ; whilst the longitudinal fibres, 

 drawing up and widening the walls of the oesophagus, over the sides of 

 the morsel of food, facilitate its descent. Gravitation, though it may 

 assist, has but little influence on the downward movement of food or 

 liquids. The resistance to be overcome is slight, consisting onty of 

 the elastic pressure of the walls of the oesophagus and of the surround- 

 ing parts. Solid substances, and even fluids, are habitually swallowed 

 by the horse and other animals, against the force of gravity ; and 

 certain clowns can perform the feat of eating and even drinking, whilst 

 "standing upon their heads." The rate of motion of food through 

 the oesophagus, is not so rapid as that through the pharynx. Ordi- 

 narily, the movement causes only a slight sensation at the upper end 

 of the oesophagus; but if the morsel be too large, the act is painful, 

 especially as the mass is passing through the diaphragmatic cesopha- 

 geal opening. As the oesophagus receives fibres coming from the 

 spinal accessory nerves, but reaching it through the pneumogastrics, 

 division of the latter in the neck paralyzes the lower part of this tube, 

 so that the food remains in it, and distends it. It also receives sym- 

 pathetic nerve- fibres. 



The three stages of deglutition are distinguished from each other in 

 a remarkable manner, according to the mode in which they are regu- 

 lated, or governed, through the nervous centre. The first stage is 

 voluntary ; we place the food between the tongue and the palate, and, 

 by an eflort of the will, pass it backwards through the fauces, into the 

 pharynx. Even the accompanying movement of the soft palate, to 

 shut off the nasal fossae, which is an associated movement, so deter- 

 mined by habit as to be unconsciously performed, is nevertheless a 



