THE MECHANICAL FACTORS OF DIGESTION 



407 



impulses from the central nervous system. Some of the movements 

 may be purely muscular in origin. 



To study the movements, animals or men are given food mixed 

 with bismuth subnitrate, or, better, with bismuth oxychloride. The 

 position and movements of the food is observed, by means of the 

 X rays and the fluorescent screen, or by almost instantaneous 

 radiographs (Fig. 199) at various intervals after the taking in of 

 the food. Tracings may be taken upon tissue paper laid upon a piece 

 of lead glass* placed over the screen (Fig. 200). The great advantage 

 of this method is that the normal passage of food through the alimen- 

 tary tract can be observed over a long period of time, and the charac- 

 teristic movements of each part, their normal rate and frequency, be 



Fia. 200. TRACINGS OF THE SHADOW CAST BY THE STOMACH (CAT), SHOWING CHANGES- 

 IK THE SHAPE OF THE ORGAN AT INTERVALS OF AN HOUR DURING THE DIGESTION 

 OF A MEAL. (Cannon.) 



accurately studied. The disadvantage of viewing the guts directly 

 after operative procedures is that the normal movements are greatly 

 interfered with thereby, or even abolished. Nevertheless, they can 

 to a certain extent be studied by immersing the anaesthetized animal 

 in a bath of warm Ringer's solution, before opening the abdominal 

 walL 



Movements of Mastication. By an up-and-down movement of 

 the lower jaw, the food is seized by the front teeth; by a side-to-side 

 movement, it is chewed by the back teeth. The tongue and cheeks 

 assist in this latter process by forcing the food between the grinding 



* The lead glass is used to protect the observer from the ill effects of prolonged 

 exposure to X rays. 



