504 SPECIAL PHYSIOLOGY. 



particle being forced upwards into the nasal fossae, where it would ex- 

 cite irritation, or downwards into the larynx, whence it would descend 

 into the windpipe, and cause coughing, difficulty of breathing, or suf- 

 focation. The posterior nares are accordingly protected by the eleva- 

 tion and tension of the soft palate above the middle portion of the 

 pharynx, in the mode already described (p. 502), so as to form an 

 inclined plane, beneath which the food glides into the pharynx, as 

 this ascends to receive it. At the same time, the opening into the 

 larynx is protected by the epiglottis, a leaf-like valve, situated at the 

 root of the tongue (p. 200), Fig. 87, e, Fig. 9, e. This valve, in the 

 ordinary condition of the parts, stands erect, with its free margin 

 directed upwards ; the larynx then communicates with the middle 

 portion of the pharynx, and air can pass from the nose, and mouth, 

 if that be open, to and from the windpipe and lungs. When, however, 

 the tongue is raised, and pressed backwards at the end of the first 

 stage of deglutition, the larynx is elevated, and the mass of food, or 

 the portion of liquid, then swallowed, presses the previously erect 

 epiglottis downwards and backwards, so as, together with certain 

 folds of the mucous membrane connected with its borders, completely 

 to close the opening into the larynx, whilst the food or drink is pass- 

 ing by it, into the lower portion of the pharynx. The moment the 

 solid or fluid has thus passed down, the tongue resumes its previous 

 position, the epiglottis is again erected by the elastic folds connecting 

 it with the anterior part of the larynx and root of the tongue, and 

 the air-passage is once more free for the purposes of respiration. 



The third stage of deglutition is performed by aid of the muscular 

 walls of the gullet or oesophagus. This musculo-membranous tube is 

 that portion of the alimentary canal, which extends from the pharynx 

 down to the stomach. It measures about nine inches in length, and is 

 the narrowest part of the alimentary canal, being itself narrowed at its 

 lower, but narrowest at its upper end. It descends through the lower 

 part of the neck and through the whole length of the thorax, and 

 then, perforating the diaphragm, opposite the ninth dorsal vertebra, 

 enters the abdominal cavity, and immediately opens into the stomach. 

 It is supported upon the vertebral column, being placed between the 

 carotid arteries, and behind the trachea, the heart, and the arch of 

 the aorta ; below the latter, it lies in the space between the two 

 pleurae, to the right ; and then in front, of the descending aorta ; it 

 traverses the diaphragm through a special opening, named the oesoph- 

 ageal opening. The walls of the oesophagus are composed of three 

 coats, muscular, areolar, and mucous. 



The muscular coat consists of an external layer of longitudinal 

 fibres, and an internal layer of circular fibres ; at the upper end of the 

 oesophagus, these fibres are chiefly striated, and striated fibres are to 

 be found in smaller numbers even down to its lower end; but; the 

 great mass of the muscular coat consists of the plain, or iinstriped, 

 muscular fibres. The areolar coat is a soft distensible tunic, which 

 supports the mucous coat. The mucous coat, reddish above, and pale 

 below, is thick, and when the oesophagus is closed, it is thrown into 

 numerous longitudinal plicae ; in this state, a section across the tube 



