84 



ELEMENTARY BIOLOGY 



having the consistency of a rather thick pea soup, and all of 

 it has passed on into the intestine. 



119. The intestines. There are two distinct parts, or divi- 

 sions, of the gut among the highest animals. The first part is 

 called the small intestine, and in human beings it is about 

 one inch in diameter and about twenty-four or twenty-five 

 feet long. The small intestine opens rather abruptly into the 

 large intestine, which is about two inches in diameter and 



about five feet long (see 



Fig. 28, y, k). 



The wall of the intes- 

 tine is rather thin and 

 soft. You have probably 

 handled a piece of pig 

 gut or calf gut, which is 

 used as sausage casing. 



In the living animal the 

 wall of the intestine is 

 not so hard and stiff as 

 we sometimes find it in 

 the sausage casing. This 

 wall is made up of 

 several layers of tissue. 

 The inner lining carries 

 very small glands, and 

 the outer layers contain 

 muscle cells. To this extent the wall of the intestine is like 

 the wall of the stomach. The muscle cells of the gut are 

 arranged in rings, so that when they contract they simply 

 reduce the diameter of the intestine at any given point. The 

 contraction starts at the forward end — that is, the end nearest 

 the stomach — and passes backward along the whole length 

 of the small intestine, aided by longitudinal muscles. As a 

 result of this wave of contraction some of the thick mixture 

 of food and digestive juices is moved along, a short distance 



Fig. 30. Glands of the stomach 



The gastric juice is poured into the stomach through 



tubes, (7, which are hned by a layer of delicate cells ; 



it is produced by special gland cells, b, from materials 



brought by the blood in fine vessels, c. ( X 200) 



