mouth. 



I^j^-cesopha^ 



-int-est-ine 



CHAPTER XIII 



THE 

 DIGESTIVE SYSTEM 



'mmi 



crop 



^i^jara 



irzjcaecxjin 



jdocccol opening 



Part of food tube of worm. 



Alimentary canal of a bird. 



What si)nUaritics and differences are found in the digestive sys- 

 tems of a frog and of a man. How is food prepared for digestion? 



We shall find in dissecting a frog and studying its digestive or- 

 gans that its digestive processes are in many ways similar to those 

 of man. 



Problem. Study of the internal organs of a frog. 



Place the frog to be studied in a covered jar. Pour a little ether into the 

 jar or put in the jar a sponge saturated \\Tith ether. When the animal is 

 dead, remove and place it, with the ventral side up and the head away from 

 you, in shallow water in an individual dissection pan. Pin the legs out so 

 that they will not interfere with the dissection. By means of forceps, lift up 

 the skin in the center of the body and cut a very small opening near the pos- 

 terior end. Insert the point of the scissors into the opening and carefully cut 

 along a median line to the mouth. Again insert the scissors near the legs and 

 cut through the muscles under the skin. Be siu-e to cut through the pectoral 

 girdle of bones near the forelegs. Insert the scissors at the anterior and pos- 

 terior ends of the median slit and cut at right angles on either side. Turn 

 back and cut off the loosened flaps of skin and the underlying muscles, in order 

 to expose the internal organs. If the specimen is a female, remove nearly all 

 the eggs so that the organs may be more readily seen. 



I. ]Make an outline dra\\'ing, natural size, of the shape of the frog. 



II. Observe the cone-shaped heart midway between the fore limbs. If the 

 heart is still beating, it does not indicate that the animal is aliv^e. It means 



114 



