DEGLUTITION. 315 



In the horse, ass, dog, sheep, and ox the lower parts of the oesoph- 

 agus are supplied, as in man and the rabbit, by the recurrent fibres of 

 the vagi ; the upper portions are, however, supplied by a long branch 

 of the pharyngeal nerve which descends in the walls of the oesophagus 

 as far as the thorax. In birds a similar state of affairs also holds. 



Deglutition may be excited by mechanical contact with the fauces 

 in an animal in which the cerebrum has been removed ; it is only 

 necessary that the medulla remain intact. 



Deglutition of liquids is performed by a mechanism which is almost 

 similar to that concerned in the deglutition of solids. The palate is raised 

 and made tense, the palatopharyngeal muscles contract, the glottis rises, 

 the epiglottis descends, the pharynx ascends, and the gullet contracts as 

 in the case of deglutition of solids, the difference mainly consisting in 

 the rapidity with which liquids are forced through the oesophagus. 



The motions of deglutition of liquids may be very rapid. Thus, in 

 the horse sixtj'-five to ninetj- motions may be made in each minute, 

 each swallow carrying one hundred and fifty to two hundred and fifty 

 grammes of liquid. 



The rapidity of deglutition varies according to the animal and the 

 nature of the food. The horse eating hay swallows thirty-five boluses 

 in fifteen minutes after having fasted for some time, and only ten or 

 twelve boluses in the same time as hunger commences to be appeased, 

 the weight of each bolus varying from fifty to one hundred grammes. 

 In swallowing liquids the horse moves the ears, advancing them at each 

 act of deglutition, at the same time closing the jaws. The masseters 

 may, therefore, be seen to move under the skin, and even the eyes to 

 move in their orbits. In ruminants during deglutition the ears either 

 remain motionless or move unequally. Rhythmical motion of these 

 organs as seen in the horse is absent in ruminants. 



The act of deglutition is performed as described above in all air- 

 breathing animals. In all, from the mammalia down to the amphibia, 

 the pharynx communicates with the nasal chambers, the cavities of the 

 ear on both sides, the mouth, larynx, and oesophagus. 



In the young kangaroo, while still retained in the abdominal pouch 

 of the mother, and in cetaceans, the upper part of the larynx is elongated 

 and projects into the posterior nares, so that during suckling the milk 

 passes down each side without any risk of entering the air-passages and 

 without interfering with respiration. 



In fishes which respire in the water by gills the pharynx has no 

 communication with the nasal passage, while the larynx and trachea are, 

 of course, absent. Hence, the pharynx is here a mere passage leading 

 from the mouth to the oesophagus, and the process of deglutition is con- 

 sequently greatly simplified. 



