THE FROG ioi 



receive principally venous blood, which it takes to the lungs and 

 the skin to be purified, whereas the more nearly arterial blood 

 comes into the carotid and systemic arches and is distributed 

 throughout the body. 



Exercise 9. Draw a diagrammatic sketch of the heart with the ven- 

 tral wall removed, showing these features. 



The Digestive System {Continued). Take out this system in the 

 following way: Lift up the liver with forceps, and with scissors 

 free its anterior border from the tissues beneath it, being careful 

 not to injure the lungs. Find the oesophagus, which joins the 

 stomach with the pharynx. Note that the lungs also join the 

 ventral wall of the pharynx. Take hold of the oesophagus with 

 forceps, lift it up, and with scissors cut across the floor of the 

 mouth in front of the lungs. 



The forward end of the digestive tract, with the lungs, being 

 thus cut loose from the body, can be bent backward. With scissors 

 cut the stomach and liver loose from the tissues beneath them ; cut 

 the mesentery by which the intestine is joined with the dorsal body 

 wall, being careful not to injure the flattened kidneys and testes or 

 ovaries, and straighten the intestine out. The entire digestive 

 tract, together with the lungs, will thus be removed from the body, 

 except at its hinder end. Extend it in the water and pin it there, 

 with the lungs attached to the pharynx, and the liver and pan- 

 creas attached to the duodenum by the bile duct. 



Exercise 10. Make a drawing of the digestive system, with the 

 lungs ; label all the parts and organs belonging to it. 



Slit open the stomach and the forward end of the intestine and 

 note the ridges on their inner surface. Cut open a lung and note 

 that it is a hollow sac with a network of ridges on the inner surface. 



The Urogenital System. The urinary and the genital organs 

 are in close union with each other, notwithstanding their differ- 

 ence in function, and are conveniently studied together. The 

 urinary organs consist of the paired kidneys, the paired excurrent 

 canals, or Wolffian ducts, the urinary bladder, and the cloaca. 



The kidneys are two large, flattened bodies which lie close to 

 the dorsal body wall in the posterior portion of the body cavity. 



