AN INTRODUCTION TO ZOOLOGY 



the granules being termed the zymogen granules. In the rabbit the 

 difference between the two types of cell is much more strongly 

 marked, and there are present in addition certain large sub-spherical 

 or ovoid cells. These cells are situated below the others upon the 

 basement membrane bounding them, and, as they are regarded as 

 being specially concerned with the production of Hydrochloric acid, 

 they are termed the Oxyntic cells. Acid is also produced in the 



gastric glands of Rana, but the cells 

 producing it are not easily distin- 

 guishable from the remainder. 



The function of the glands is to 

 secrete a fluid, the gastric juice, 

 that plays a very important part 

 in the digestion of the food. In 

 addition to the Hydrochloric acid 

 already mentioned, it contains certain 

 other substances that are very active 

 chemically. They are termed fer- 

 ments, or preferably enzymes, a 

 name given to substances produced 

 by living matter that are capable of 

 producing chemical changes in other 

 substances with which they are in 

 contact while they remain unaltered 

 in themselves. The enzymes appear 

 to be derived from the zymogen 

 granules which undergo a certain 

 change when discharged from their 

 cells, and their action on the food will 

 be considered later in dealing with 

 digestion. 



The spaces between the gland 

 tubules are filled with connective 

 tissue, which also extends beyond 

 them and forms a fairly thick coat, 



the sub-mucosa, plentifully supplied with blood-vessels. This 

 connective tissue is interrupted by a very thin double circle 

 of muscles, the muscularis mucosae, which follow the outline of 

 the mucous membrane. The inner part of the band is formed by 

 circular muscles, i.e. muscles whose fibres run around the stomach, 

 and the outer part by fibres at right angles to them, the longi- 

 tudinal muscles. Outside the sub-mucosa is the large muscular 

 coat of the stomach, also consisting of two parts, an inner thick 

 layer of circular muscles and an outer much thinner layer of 



FIG. 17. Peptic gland from the 

 mucous membrane of the 

 stomach. Highly magnified. 

 From Gray. 



