Chap. 34 



AMPHIBIANS 



697 



Nasal covify 



Brain 



Testis 



Adrenal gland 

 Kidney 



Urinory 

 bladder 



left leg 



Pancreas' Small intestine 



Fig. 34.16. Frog showing the relative positions of systems. 



able to pass through cell membranes as they could not have done before. 

 Finally simple sugars, fatty acids, glycerol and amino acids are absorbed 

 through cells in the intestinal lining. The fats are taken up by the lymph, the 

 sugars and amino acids by the blood plasma, and all are distributed by these 

 fluids. The vertebrate liver is only indirectly a digestive gland (Fig. 34.16). 

 It is an excretory organ that picks waste substances from the blood and pre- 

 pares them for elimination, the nitrogenous waste into urea and the pigment 

 of worn-out red blood cells into bile pigments. It is a storage place for an 

 emergency food (glycogen). It produces bile that carries away waste pigments 

 and certain other waste products and performs important functions in the in- 

 testine connected with the digestion and absorption of fat. Bile aids digestion 

 indirectly because it stimulates the enzymes of the pancreatic juice by creating 

 the alkaline environment necessary for them to act. It is a lubricator and easy 

 slipping is essential. Excess bile is stored in the gall bladder. The liver is in 

 short a strainer and balancer of the blood content, having also an indirect but 

 essential part in digestion. 



Peristaltic contractions gradually move the undigested residue of the food 

 into the large intestine. Its walls absorb water from this, contract upon it, and 

 eventually force it into the cloacal chamber and out of the body through the 



