:U20 THE DOG-FISH 



The spiral valve is a membranous fold, attached 

 along one edge to the inner surface of the intes- 

 tine, round which it runs spirally. The fold is 

 about an inch and a half wide at the anterior 

 end, but gradually diminishes in width towards 

 the posterior end, and disappears at the com- 

 mencement 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 colon. The bile-duct has a 

 total length of about three inches. 



Find the duct along the edge of the fold of mesentery 

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

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

 blue : follow it forwards to the liver and gall-bladder, a/nd 

 backwards to its opening into the intestine. 



2. The pancreas is a whitish, laterally compressed body 



about an inch and a half long, lying in the angle 



