THE FROG 43 



of the Frog, p. 281). If possible observe in a living frog 

 the beating of the posterior pair of lymph hearts; these are 

 situated just beneath the skin in a slight depression between 

 each hip-joint and the dorsal median line. The contraction 

 of these hearts usually causes a slight pulsation of the skin, 

 but it may be seen more distinctly in a freshly-killed speci- 

 men from which the skin of this region has been removed 

 without injury to the hearts. 



VIII. THE RESPIRATORY SYSTEM. 



Respiration in the frog is not carried on exclusively by 

 means of the lungs; the skin and the mouth cavity are im- 

 portant accessory organs of respiration. 



The lungs communicate with the pharynx or posterior 

 part of the mouth cavity by means of the glottis, which opens 

 from the dorsal surface of a cartilaginous box, the larynx, 

 whose cavity is continuous with the cavities of the lungs. 



Returning to the study of your original specimen, remove 

 the liver, taking care not to injure the adjacent parts. Re- 

 move the skin and muscle from the throat region and expose 

 the hyoid apparatus, the body of which consists of a thin 

 shield-shaped sheet of cartilage, lying just in front of the 

 larynx. This cartilage serves for attachment of the tongue 

 and several muscles that move the floor of the mouth; a pair 

 of strong bony processes, the thyrohyoid processes, ex- 

 tend posteriorly and support the larynx which lies between 

 them. After examining a preparation of the hyoid, larynx 

 and lungs, remove these organs as follows : cut the oesoph- 

 agus across about 5 mm. behind the base of the lungs and, 

 inserting one blade of the scissors in the open end, cut for- 

 ward along the side toward the angle of the jaw and dorsal 



