DIGESTIVE SYSTEM. 235 



of the spiral valve, and turn it over to the left side, so as to 

 fully expose the cavity of the intestine. Wash freely. 



The spiral valve is a membranous fold, 

 attached along one edge to the inner surface of 

 the intestine, round which it runs spirally. The 

 fold is about an inch and a half wide at its 

 anterior end, but gradually diminishes in width 

 towards the posterior end, and disappears at the 

 commencement of the rectum. The first turn 

 of the spiral is a long one, but the succeeding 

 ones, of which there are usually seven or eight, 

 are only about a quarter of an inch apart. The 

 general appearance of .the valve is that of a 

 series of cones one within another, the apices of 

 the cones. being usually directed forwards, but 

 sometimes at the hinder end backwards. 



The valve serves to retard the passage of food 

 down the intestine, and to increase the extent of 

 its absorptive surface. 



B. The Glands. 



1 . The liver has been described above. 



a. The gall-bladder is a large irregular sac imbedded 



in the anterior part of the left lobe of the liver, 

 close to the median plane. 



b. The bile-duct leaves the gall-bladder between the 



two lobes of the liver, and almost in the median 

 plane. It receives ducts from the lobes of the 

 liver, and runs back along the ventral margin of 

 the mesentery to the intestine, along which it 

 runs for a short distance to open into the com- 

 mencement of the ileum. The bile-duct has a 

 total length of about three inches. 



Find the duct along the edge of the fold of mesentery 

 hetiveen the liver and the intestine : make an incision in its 

 ivalls, and inject it with a coloured fluid such as Prussian 



