DEGLUTITION. 307 



developed. In the case of tine serous cells the changes are not so 

 readily recognizable, since the microscopic changes are less marked, but 

 the probability is that the same sort of processes occur. In the actual 

 formation of the secretion we have thus two processes concerned. We 

 have the development of mucin in the muciparous cells, and of ptyalin. 

 During activity, from dilatation of the capillaries, the blood-serum, more 

 or less modified in composition, reaches the acini, and from there passes 

 into the glandular cells, while at the same time the fluid filters from these 

 cells into the duct, and so constitutes secretion. The inorganic con- 

 stituents of secretion are, therefore, removed from the blood by a simple 

 process of osmosis, or filtration, while the organic constituents are the 

 results of active manufacturing processes occurring within the proto- 

 plasmic cell-contents. 



V. DEGLUTITION. 



By the term deglutition is meant the various co-ordinated muscular 

 movements which result in the passage of the food from the mouth to 

 the stomach. 



The act of deglutition m&y be divided into three different stages. 

 In the first stage, which occurs in the mouth, the bolus of food passes 

 to the isthmus of the fauces, in the second stage it passes through the 

 pharynx, and in the third stage it traverses theO3sophagus. 



When the food has been sufficient!}' masticated it is gathered into a 

 bolus by the contraction of the muscles of the tongue, the tip of the 

 tongue being raised b}>- the intrinsic muscles of the tongue, aided by the 

 stylo-glossus,and the bolus passes back between the tongue and the hard 

 palate to the anterior portions of the fauces (Fig. 129). This transferring 

 of food from the mouth to the pharynx occurs when the teeth are in 

 contact, since the jaws must be closed to afford support to the hyoid 

 muscles, which we will find to be concerned in the later steps of the 

 process, and in the herbivorous animals is accomplished so rapidly that 

 110 more marked duration of closure of the jaws can be detected than at 

 any other time. When the bolus is very large mastication ceases at the 

 moment of deglutition, as in carnivora and other animals that swallow 

 the entire contents of the mouth at one movement. This first stage of 

 deglutition is entirely within the control of the will, and may be pro- 

 longed or accelerated, and the movements of the bolus are perceptible 

 to the sensory nerves of the part. When the bolus has once been placed 

 upon the dorsum of the tongue, the tip, middle, and root of the tongue 

 are successively pressed against the hard palate, and the contents of the 

 mouth are thus propelled toward the pharynx ; an active contraction of 

 the mylo-hyoid muscles then takes place, as may be recognized by the 

 finger placed below the lower jaw, the dorsum of the tongue is raised up, 



