3 i4 MECHANISMS OF THE DIGESTIVE TRACT. 



impulses travel by the motor root of the fifth cranial nerve to the 

 elevators of the jaw and the mylohyoid, along the seventh nerve to the 

 digastric and muscles of the lips and cheek, and along the hypoglossal 

 nerve to the muscles of the tongue. 



SUCKING. 



During the first year of life, before the development of the teeth, 

 the child is dependent for its sustenance on liquid food, which it obtains 

 under normal circumstances from the breast of the mother. In this 

 act the lips enclose the nipple, adherence being assisted by the exist- 

 ence of a special membranous projection of the front part of the 

 gums of both jaws. During suction, the respiration goes on through 

 the nose without check. Normally the dorsuni of the tongue lies 

 against the hard palate, but, according to Bonders, 1 at the back part it 

 is separated from the soft palate by a small space. Owing to the 

 weight of the jaw, there is a negative pressure in this space of 2-4 mm. 

 Whether such a space is present under ordinary conditions or not, it is 

 certain that the jaw is maintained in its position, not by muscular 

 effort, but by the pressure of the air ; so that, if a tube from a mano- 

 meter be passed between the tongue and the palate, the manometer 

 shows a slight negative pressure corresponding to the \veight of the jaw. 

 If, therefore, the nipple of the breast or feeding-bottle be passed between 

 the tongue and palate, there will be a tendency for the fluid to flow into 

 this space. During active sucking the space is enlarged and the 

 negative pressure increased by means of movements of the tongue, this 

 organ being retracted and also hollowed out on its upper surface by 

 means of the genioglossi and palatoglossi. 



DEGLUTITION. 



Muscular mechanisms. Since the time of Magendie, physio- 

 logists have been accustomed to divide the act of swallowing into three 

 stages. 



In the first stage, the bolus of food is carried past the anterior 

 pillars of the fauces ; the second stage comprises the period during which 

 the food is conveyed through the pharynx, past the openings of the 

 nasal cavities and larynx ; and during the third stage it is passed 

 through the lower part of the pharynx and down the oesophagus. 

 Since, however, there is no pause between these various stages, but the 

 muscular movements involved follow one another continuously, it is 

 better to consider the act of deglutition as one process. 



The food, when sufficiently masticated, is collected as a bolus on the 

 dorsum of the tongue by the movements of this organ. A pause then 

 occurs in the process of mastication, a slight movement of the diaphragm 

 usually takes place (" respiration of swallowing "), and a sudden elevation 

 of the tongue occurs, which travels from the apex to the base, so squeez- 

 ing the bolus backwards past the isthmus faucium. At this moment 

 the bolus has arrived at a part of the pharynx which is common to the 

 functions of respiration and deglutition. It has therefore to be con- 

 veyed quickly past the openings of the air passages into the pharynx, 



1 Arch.f. d. ges. Physiol. Bonn, 1875, Bel. x. S. 91. 



