76 THE FROG chap. 



going waste, and in consequence needs continual repair. 

 The waste-matters chiefly take the form of carbon dioxide, 

 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 substances 

 gradually disappear from the canal, and are absorbed into 

 the system. Finally, the indigestible constituents are ex- 

 pelled as fseces. 



Our next task must be to learn something of the process 

 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 Dig^eStive 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 Hie postcaval vein (Figs. 3 and 4, pt. cv), Ihe hepatic portal 

 vein (Fig. 3, hp. pt), the aorta (Fig. 4, d. ao), and the splanchnic or 

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

 not to injure the aorta, remove the greater part of the digestive 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 dis- 

 secting dish as nearly as -possible in their natural position. Turn the 



