300 ESSENTIALS OF PHYSIOLOGY. 



slightly backwards by the contraction of the stylo-glossi and palato- 

 glossi. In this way the bolus is propelled through the opening bounded 

 by the anterior pillars of the fauces, beyond which the act becomes 

 involuntary. The later stages are subdivided anatomically into 

 pharyngeal and oesophageal, but physiologically they are to be looked 

 upon as a continuous series of reflexes. 



While the bolus is in the pharynx, the soft palate is raised so as to 

 form an inclined plane and prevent the passage of food into the nares. 

 At the same time the opening into the respiratory tract is guarded by 

 the elevation of the larynx under the posterior end of the retracted 

 tongue, by the constriction of the aperture of the larynx itself, and by 

 closure of the glottis. The arytenoid cartilages are rotated inwards by 

 contraction of the lateral crico-arytenoid muscles, and approximated by 

 contraction of the arytenoideus. They are at the same time drawn 

 forward by contraction of the thyro-arytenoids, so that the glottis takes 

 the form of a T-shaped slit. The opening of the larynx is diminished in 

 size by contraction of the ary-epiglottidean muscle fibres. At the same 

 time, by the elevation of the larynx and the drawing back of the tongue 

 the opening is further guarded by the lower part of the epiglottis. 



The passage of the bolus through the oesophagus is effected by 

 a wave of contraction preceded by relaxation in the muscular wall 

 of the tube. The oesophagus consists of four coats: (1) a mucous 

 coat, bounded externally by a layer of smooth muscle fibres arranged 

 longitudinally, the muscularis mucosw, and lined by stratified squamous 

 epithelium ; (2) a submucous coat of loose connective tissue containing 

 mucous glands, which supply a lubricating fluid ; (3) a muscular 

 coat, composed of striated muscle in the upper part of the tube 

 and of smooth muscle in the lower part, and consisting of an outer 

 longitudinal and inner circular layer ; and (4) a fibrous coat. Owing to 

 the more rapid contraction of the striated muscular fibres, the bolus travels 

 more quickly in the upper part of the oesophagus than in the lower part. 



The time taken by the act of deglutition is normally five or six 

 seconds. This may be ascertained by listening over the pharynx or 

 the oesophagus by means of a stethoscope, the entrance of the food 

 into the pharynx at the beginning of the act, and again into the stomach 

 at its close, being characterised by distinct sounds. A more precise 

 method is direct observation with the aid of Rontgen rays, the food 

 swallowed in this case being mixed with bismuth so that it gives a well- 

 marked shadow. 



During swallowing there is temporary inhibition of respiratory 

 movements, and in this way a further safeguard is introduced against 

 the entrance of food particles into the larynx. 



