38 AN INTRODUCTION TO ZOOLOGY. 



cal in chemical composition with a substance which is 

 found in old blood-clots. The liver, therefore, has a, 

 double work to do : to cleanse and filter the blood, 

 and to make something new and useful out of the 

 waste residue which it has withdrawn from the blood. 

 Hence it is a very important organ, and is connected 

 with very important blood-vessels. 



All these organs belonging to the alimentary canal 

 are lined with the peculiar cells of the digestive layer. 

 To save room, the cells are grouped in little pockets, 

 which are called glands ; when they are very long, us 

 in the intestine, they are called villi (Lat. villus, a 

 hair, pi. villi). The secreting cells consist of a clear 

 transparent protoplasm, which is very active in its 

 chemical changes, becoming full of fat granules as 

 soon as it absorbs anything. 



These cells are also the channels of absorption. 

 They suck in the fluid products of digestion, and 

 transfer them to the lacteals of the lymphatic system 

 (see p. 45). 



Besides these, the mouth is provided with glands 

 called salivary glands ; the moisture of the mouth, 

 which they secrete, is called saliva, and contains a 

 ferment called ptyalin, which digests starch, and 

 changes it into sugar. The secretion of the stomach, 

 called gastric juice, contains hydrochloric acid, and 

 a ferment called pepsin, which digests proteid food 

 (see p. 8) ; the ferment of the pancreatic gland 

 is called trypsin ; as already stated, it helps to 

 digest both. In this it aids the juice secreted by the 



