DEGLUTITIOK 219 



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. After mastication has been 

 completed, 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 appreciate, 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 accurately 

 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 experiments of Panizza, which have already been referred to 

 in connection with mastication, it was found that paralysis of the tongue by section of 

 the hypoglossal nerves in dogs deprived the animals of the power of swallowing, even 

 when a bolus of meat or bread was put upon its dorsal surface. In an observation on a 

 young dog, in which we divided both hypoglossal nerves, the effect upon deglutition was 

 very marked. The animal ate with difficulty, the pieces of meat which were given him 

 frequently dropping from the mouth. He was able to swallow only by jerking the head 

 suddenly upward, so as to throw the meat past the base of the tongue ; and, even when 

 deglutition commenced, the first steps took place slowly and with apparent difficulty. 

 The process of drinking was very curious. The animal made the usual noise in attempt- 

 ing to drink, but the tongue did not come out of the mouth, and the only way he seemed 

 to get any water was by jerking the head and moving the jaw so as to throw some of the 

 liquid into the mouth. On causing him to drink from a graduated glass, it was found 

 that he drank four fluid ounces in four minutes. In the case of a young girl, reported to 

 the Academy of Science, in 1718, by De Jussieu, 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 portion of its base gen- 

 erally remains sufficient to press against the palate and thus act in the first period of deg- 

 lutition. 



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

 are generally involuntary. When the food has been sufficiently 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 imperative necessity of air in the system must, in a 

 short time, overcome any voluntary effort by which respiration has been arrested. 



The second period of deglutition involves more complex and important muscular 

 action than the first. By a rapid and almost convulsive series of movements, the food is 

 made to pass through the pharynx into the oesophagus. The movements are then entirely 

 beyond the control of the will, and belong to the kind usually called reflex. After the 

 alimentary mass has passed beyond the isthmus of the fauces, it is easy to observe a sud- 

 den and peculiar movement of elevation of the larynx by the action of muscles which 



