722 PHYSIOLOGY 



or piston recorders. This method has been employed both in men and 

 in animals. 



When a mouthful of water is taken two sounds may be heard on ausculta- 

 ting over the oesophagus. The first sound immediately follows the beginning 

 of the act of swallowing and is probably due to the impact of the fluid 

 against the posterior pharyngeal wall, brought about by the sudden con- 

 traction of the mylohyoid and other muscles which throw the fluid from the 

 back of the tongue across the pharynx. The second sound is heard best by 

 listening over the epigastrium. It begins from four to ten seconds after the 

 first sound, and lasts for two or three seconds. The interval between the 

 two sounds is not constant, and may vary in the same individual. If the 

 observation be carried out on a man lying on his back, the trickling second 

 sound is changed into a series of sounds which have been described as squirts, 

 which vary from two to five in number, each lasting about one second. The 

 second sound may be absent when a solid bolus is swallowed. On observing 

 the process by Rontgen rays, very much the same time-relations are obtained. 

 If a mouthful of milk mixed with bismuth carbonate be swallowed, it will 

 be seen passing rapidly down the oesophagus to the cardiac orifice of the 

 stomach. Here the passage becomes slow, and the fluid escapes slowly in a 

 narrow stream into the stomach. The average time which elapses between 

 the beginning of deglutition and the disappearance of the last trace of fluid 

 from the oesophagus is about six seconds. The same course of events is 

 induced when the food swallowed is semi-solid. If however the bolus be 

 dry, such as a cachet of bismuth carbonate, it may pass down the oesophagus 

 with extreme slowness and may take as much as fifteen minutes to reach the 

 cardiac orifice, although the individual who has swallowed it is quite unaware 

 of its continued presence in the oesophagus. If, as would normally be the 

 case, the cachet be well moistened with saliva or water before swallowing, 

 it passes much more rapidly, the total time taken being between eight and 

 eighteen seconds. 



It has been customary since the time of Magendie to divide the act of 

 swallowing into three stages : during the first the bolus of food is carried 

 past the anterior pillars of the fauces; during the second through the 

 pharynx, past the openings of the nasal cavities and of the larynx; and 

 during the third through the oesophagus into the stomach. There is how- 

 ever no pause between these various stages. The act of deglutition is one, 

 and the initiation of the first stage inevitably involves the completion of the 

 third. The food is masticated, and is collected as a bolus on the dorsum 

 of the tongue. A pause then takes place in the movements of mastication, 

 a slight movement of the diaphragm usually occurs known as c respiration of 

 swallowing/ and then a sudden elevation of the tongue throws the bolus back 

 through the anterior pillars of the fauces. In this movement the chief 

 factor is the contraction of the mylohyoid muscle, which presses the tongue 

 against the palate and pushes it backwards. The backward movement of 

 the tongue may also be aided by the contraction of the styloglossus and 

 palatoglossus muscles which pull the base of the tongue suddenly backwards. 



