52 The Vertebrate Organ Systems 



These are usually lighter in color than the rest of the dorsum. At the 

 anteriormost portion of the head is the large transverse mouth; at the 

 posterior portion of the body is the anus. There is no tail in the 

 adult frog. 



Fig. 7. — The leopard frog, Rana pipiens, showing regions. 



TBAHSVfRSE PLANE 



MEDIAN fLANE 



FRONTAL PLANE 



Fig. 8. — Explanation of planes or cuts possible for study. 



The Head Region. — Near the tip of the snout above the mouth 

 are two small openings, the external nares or nostrils. Air passes 

 through these during respiration. On the dorsal lateral portion of each 

 side of the head is the large eye. Each eye is protected by an upper 

 and lower eyelid. When these are closed, the eye is retracted into the 

 mouth cavity. Further protection is given to the eye by the third 

 eyelid, the nictitating membrane, which is located at the corner of the 

 eye and moves transversely across the eyeball. This eyelid is trans- 

 parent and may be seen as it flicks rapidly across the eye. Posterior 

 to each eye is a flat circular membrane, the eardrum, or tympanic mem- 

 brane. Sound waves strike this membrane and the vibrations are trans- 

 mitted into the inner ear. 



Appendages. — Attached to the ventral lateral portion of the trunk 

 are the two pairs of legs. The forelegs are very short ; each is com- 

 posed of an upper arm, forearm, wrist, and hand. The hand has four 



