206 DIGESTION MASTICATION, INSALIVATION, DEGLUTITION. 



tube is distended. Its epithelium is thick, of the squamous variety, and is 

 continuous with and similar to the covering of the lower portion of the 

 pharynx. It is provided with papillae of the same structure as those found 

 in the pharynx, the conical variety predominating. Small, racemose glands 

 are found throughout the tube, forming, by their aggregation at the lower 

 extremity just before it opens into the stomach, a glandular ring. 



Mechanism of Deglutition. For convenience of description, physiologists 

 have generally divided the process of deglutition into three periods. The first 

 period is occupied by the passage of the alimentary bolus backward to the 

 isthmus of the fauces. This may appropriately be considered as a distinct 

 period, because the movements are effected by the action of muscles under 

 the control of the will. The second period is occupied by the passage of the 

 food from the isthmus of the fauces, through the pharynx, into the upper 

 part of the oesophagus. The third period is occupied by the passage of the 

 food through the oesophagus into the stomach. 



In the first period the tongue is the important agent. At the beginning 

 of this period, the mouth is closed and the tongue becomes slightly increased 

 in width, and with the alimentary bolus behind it, is pressed from before 

 backward against the roof of the mouth. The act of swallowing is always 

 performed with difficulty when the mouth is not completely closed; for the 

 tongue, from its attachments, must follow, to a certain extent, the movements 

 of the lower jaw. The first part of the first period of deglutition, therefore, 

 is simple ; but when the food has passed beyond the hard palate, it comes in 

 contact with the hanging velum, and the muscles are brought into action 

 which render this membrane tense and oppose it in a certain degree to the 

 backward movement of the base of the tongue. This is effected by the action 

 of the tensor-palati and the palato-glossus. The moderate tension of the soft 

 palate admits of its being applied to the smaller morsels, while the opening 

 is dilated somewhat forcibly by masses of greater size. 



It is easy to see, in analyzing the first period of deglutition, that liquids 

 and the softer articles of food are assisted in their passage to the isthmus of 

 the fauces by a slight suction force. This is effected by the action of the 

 muscles of the tongue, elevating the sides and depressing the centre of the 

 dorsum, while the soft palate is applied to the base. 



The importance of the movements of the tongue during the first period 

 of deglutition is shown by experiments on the inferior animals and by cases 

 of loss of this organ in the human subject. In the case of a young girl, 

 reported by De Jussieu (1718), in which there was congenital absence of the 

 tongue, deglutition was impossible until the food had been pushed with the 

 finger far back into the mouth. In cases of amputation of the tongue, a por- 

 tion of its base generally remains, which is sufficient to press against the 

 palate and thus act in the first period of deglutition. 



The movements in the first period of deglutition are under the control of 

 the will but are generally automatic. When the food has been thoroughly 

 masticated, it requires an effort to prevent the act of swallowing. In this 

 respect, the movements are like the acts of respiration, except that the imper- 



