DIGESTION AND ABSORPTION 



33i 



Glands. — In addition to the alimentary canal proper, we find a 

 number of digestive glands, varying in size and position, connected 

 with the canal. A gland is a collection of cells which takes up 

 materials from the blood and pours out the secretion as a fluid; 

 such cells, together with the blood vessels and nerves passing to 

 them, are held in place by a web of connective tissue. 



It is the substances formed by these glands that cause the di- 

 gestion of food. The substances secreted by the cells of the glands 

 and poured out into the food tube act upon insoluble foods so as 

 to change them to a soluble form. 



Structure. — The entire inner surface of the food tube is covered with a 

 soft lining of mucous membrane. This is always moist because certain cells, 

 called mucous cells, empty out their con- 

 tents into the food tube, thus lubricating 

 its inner surface. When a large number 

 of cells which have the power to secrete 

 or form fluids are collected together, 

 the surface of the food tube may be- 

 come indented; the little depression 

 thus formed is a simple gland. If such 

 a tube is greatly branched, with one 

 common duct or tube connecting it 

 with the inside surface of the food tube, 

 it is then called a compound gland. If 

 we think of a very sour pickle or a de- 

 licious bit of candy, our mouth waters. 

 This is caused by the action of certain 

 nerves upon some of the gland cells in 

 the mouth (salivaric glands) ; this re- 

 sults in the setting free of a fluid we 

 call saliva. In case of stage fright, the 

 secretion of saliva is prevented by the action of the nervous system, and 

 fche mouth becomes dry. 



Comparison of the Alimentary Canal of a Frog with that op 

 Man.^ — (Material — frogs preserved in alcohol or four per cent formol.) 

 Notice the shape and size of the mouth when closed and when opened. 

 Look for teeth. Feel with your finger the upper and lower jaws in the 

 roof Oi" the mouth. The prominent teeth on the roof of the mouth are 

 known as the vomerine teeth. Notice the mucous membrane lining in 

 the interior of the mouth. With a pencil or tweezers, find the baglike 

 openmg of the gullet through which food passes to the stomach. Do not 



Structure of glands; 1, simple pit, sur- 

 rounded by capillaries; 2, flask- 

 shaped gland, with short duct ; 3, 4, 

 more complex glands, with longer 

 ducts. 



' For more detailed work, see Hunter and Valentine, Manual, pp. 174-177. 



