76 THE FROG CHAP. 



oxide, water, and urea. Repair is effected partly by 

 breathing (see Chapter IX), partly by feeding. Food, 

 consisting of proteids, fats, carbohydrates, inorganic 

 substances, as well as water, is taken into the enteric 

 canal, where by the action of the three digestive juices 

 gastric juice, pancreatic juice, and bile it is converted 

 partly into a solution (peptones, sugar, fatty acids, 

 glycerine), partly into an emulsion (fats) . As it is driven 

 along the canal, from stomach to rectum, by the action 

 of the muscular coat, the dissolved or emulsified sub- 

 stances gradually disappear from the canal, and are 

 absorbed into the system. Finally, the indigestible 

 constituents are expelled as faeces. 



Our next task must be to learn something of the pro- 

 cess of absorption, and of the means by which the 

 digested food is conveyed to the various parts of the 

 body, so as to supply them with the means of repairing 

 the waste they are constantly undergoing. For this 

 purpose we must now study what is called the vascular 

 system, i.e., the heart, the blood-vessels, and the various 

 cavities containing lymph. 



PRACTICAL DIRECTIONS 



The Digestive Organs. Pin down under water, with the 

 ventral side uppermost, as before, the specimen already 

 dissected, or another in which the body-cavity has been 

 opened in the same way. Note the positions of the postcaval 

 vein (Figs. 3 and 4, pt. cv), the hepatic portal vein (Fig. 3, 

 hp. pt), the aorta (Fig. 4, d. ao), and the splanchnic or coeliaco- 

 mesenteric artery (Figs. 3 and 4, ccel. mes). Then, taking care 

 not to injure the aorta, remove the greater part of the diges- 

 tive organs, including the liver, by cutting through the 

 gullet and rectum and severing the mesentery, cutting 

 through the postcaval also where it enters the liver. (The 

 cloaca will be examined at a later stage.) Pin the organs 

 in the dissecting-dish as nearly as possible in their natural 

 position. Turn the lobes of the liver forwards (i.e., towards 



