THE FROG 



EXTERNAL ANATOMY 



In the entire animal notice the shape of the body. Is there 

 a distinct neck? From your study of the skeleton explain the 

 'hump' on the back. Where is the ankle? How many toes on 

 each foot? Are all of the feet webbed and how far does the 

 web extend along the digits? Is the anus dorsal or ventral in posi- 

 tion? Examine the surface of the body with a hand lens; do 

 any parts appear to be markedly glandular? Do you find any 

 signs of scales? 



Examine the head for eyelids and external nostrils (nares). 

 Are there fleshy folds about the nares? See the circular area, 

 the tympanic membrane on either side of the head behind the 

 eye. Note the gape of the mouth; is it large or small? Are 

 there teeth in both jaws? Look in the median line between or 

 a little in front of the eyes for a dark spot, the brow spot or frontal 

 organ. 



Draw a dorsal view of the frog, natural size. 



Cut through the tympanic membrane at its border and free 

 it from the ring (tympanic annulus) on which it is stretched. 

 This lays open the tympanic cavity, shaped like a flat funnel and 

 ending below in a large short Eustachian tube. Attached to the 

 inner surface of the tympanic membrane is a slender bone, the 

 columella auris. Where is the other end of this element? Note 

 that the columella is held in position by a thin membrane, the 

 plica plectri. Would the existence of this tend to show that the 

 columella was morphologically outside the tympanic cavity? 

 From which side has it extended into the cavity? Probe the 

 Eustachian tube ; open the mouth and see where the probe appears. 



Draw the tympanic cavity hi diagrammatic section, showing 

 tympanic membrane, columella, Eustachian tube, etc. 



INTERNAL STRUCTURE 



Cut through the ventral skin (the skin only) from the hind 

 limbs forward to the symphysis of the lower jaw and carefully 

 lift it up. Is it free from the underlying muscles at all points? 

 The free portions correspond to subcutaneous lymph spaces. 



MUSCLES. On either side of the median line in the abdominal 

 region is the large rectus abdominis muscle, divided transversely 



