170 VERTEBRATES 



ovaries in the female. Trace the connection of each of these 

 with other systems of the body, and with the cloaca. Find 

 the oviducts, irregular, somewhat convoluted tubes through 

 which the eggs pass to the cloaca. Is this connected with 

 the ovaries? If not how do the eggs reach the ovaries in 

 passing out of the body? 



V. Respiratory System. — (i) Remove the lungs from 

 the body, following them up to the mouth and being careful 

 not to injure the larynx. Notice how the larynx fits into the 

 V-shaped hyoid cartilage. Draw outer view of lungs. 



(2) Open lungs and note the structure. Can you trace 

 the plan of circulation in the walls of the cells or chambers ? 

 How is the blood aerated? Have you studied another 

 method of cleansing blood in the frog? Compare the pro- 

 cesses of respiration through the skin and through the 

 lungs. Do you see anything about the circulatory and res- 

 piratory systems to make the frog cold blooded and slug- 

 gish? 



VI. Nervous System. — (i) After removing the ab- 

 dominal viscera just referred to, there may be seen white 

 nervous cords on the dorsal side. Observe them closely. 

 Are they arranged in pairs? Trace them from their origin 

 in the vertebral column to the various parts of the body 

 to which they go. Note the difference in size. Do they go 

 singly to the body? A union of two or more nerves forms 

 a plexus. How many do you find ? One near the posterior 

 end is called the sciatic plexus. Trace the nerves to their 

 union into the sciatic nerve which supplies the legs. The 

 one at the front legs is called the brachial plexus. Begin- 

 ning with the nerve in front of the brachial plexus, count 

 the nerves toward the posterior end. They are known by 



