The Frog 47 



of the body are moved by the action of muscles, which in turn are 

 innervated ( ) by nerves. To know the internal 



structure of an animal one must know all that can be known in regard 

 to the following systems : 



1. The Digestive System. 



2. The Circulatory System. 



3. The Respiratory System. 



4. The Excretory System. 



5. The Nervous System. 



6. The Skeletal System. 



7. The .Muscular System. 



8. The Reproductive System. 



After an incision is made along the mid-line (Fig. 6) of the ventral 

 ( ) surface of the animal from the lower angle of 



the jaw to its most posterior end, and the skin-coverings are pulled aside, 

 the internal organs are seen. These are called the viscera ( ). 



The cavity in which they are found is known as the coelom, ( ) 



or body cavity. 



If the animal has just been chloroformed, the heart will still be 

 beating. The heart is contained in a sac-like structure called the 

 pericardium ( ) . 



Surrounding at least a portion of the pericardium, are three promi- 

 nent lobes of the reddish-brown liver, while the lungs, looking like small 

 strawberries, lie, one on each side, near the anterior end of the abdomi- 

 nal cavity. 



The stomach and the coiled intestine attached to it, are easily recog- 

 nizable. 



The kidneys are flattened reddish bodies attached to the dorsal body 

 wall. 



If it is the breeding season, and the frog is a female, almost the 

 entire body-cavity may be filled with thousands of eggs. The eggs 

 in turn are contained in a film-like covering known as the ovary 

 ( ) and oviducts ( ), the latter 



organs serving as tubes through which the eggs leave the body. If 

 the specimen is a male, the two testes ( ) will be 



suspended by little membranes at the side of the digestive canal (t, Fig. 

 6). The entire lining of the abdominal cavity in all the higher forms 

 of animals is called the peritoneum ( ). When one 



or two layers of this peritoneum suspend, or hold up an organ, such as 

 the digestive canal and the reproductive organs, such suspending peri- 

 toneum is called a mesentery ( ). 



THE DIGESTIVE SYSTEM 



It will be noticed that the tongue is extensile ( ), 



that is, it can be thrown forward and outward. On the tongue there 

 is secreted a sticky substance which causes objects with which it comes 



