180 DIGESTION 



ated when the tube is distended. Its epithelium is thick, of the squa- 

 mous 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 predominat- 

 ing. Racemose mucous glands are found throughout the tube, forming, 

 by their aggregation at the lower extremity just before it opens into the 

 stomach, a glandular ring (see Plate IV, Fig. 2). 



Mechanism of Deglutition. For convenience of description, physi- 

 ologists usually have 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 begin- 

 ning 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 swal- 

 lowing is always performed with difficulty when the mouth is not com- 

 pletely 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 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 movements in the first period of deglutition are under the con- 

 trol of the will but usually are automatic. When the food has been 

 thoroughly masticated, it requires an effort to prevent the act of swal- 

 lowing. In this respect, the movements are like the acts of respiration, 

 except that the imperative necessity of air in the system must, in a short 



