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I. GENERAL SURVEY OF THE FROG 



It is believed that the best introduction to the science of zoology is the 

 thorough study of a single, relatively complex animal. For this reason, we shall 

 first examine in detail the anatomy, the physiology, and the microscopic struc- 

 ture of one of the commonest animals, the frog. After learning how such an 

 animal is constructed and how it employs the structures which it possesses, we 

 shall be in a position to understand the make-up of other animals, and to appre- 

 ciate by what changes and alterations these have been built up from an extremely 

 simple starting-point. 



A. KILLING THE FROG 



The frog may be killed either by placing it for 15 to 20 minutes in a closed 

 vessel with a wad of cotton soaked in ether or chloroform, or by the method of 

 pithing. In pithing, a blunt instrument is thrust through the space between 

 the posterior end of the skull and the beginning of the vertebral column and the 

 nervous system is destroyed. To pith a frog, grasp the animal firmly in the 

 left hand and bend the head down by placing the forefinger across the snout. 

 With the finger or a blunt instrument feel for the depression between the posterior 

 end of the skull and the first vertebra (it is located at about the level of the fore 

 limbs). Cut through the skin at this place with a scissors, and press firmly 

 upon the depression with a blunt instrument until the instrument breaks through 

 the muscles into the cavity of the skull. Then thrust the instrument forward 

 into the brain and then backward into the spinal cord, moving it about so as to 

 mash the nervous system thoroughly. The best instrument for pithing is a 

 stout wire hairpin or a blunted hatpin. If the pithing is properly done, spon- 

 taneous movements cease. 



After either pithing or etherization, it will be noted that many of the activities 

 of the frog continue; the heart keeps on beating for a considerable length of 

 time and movements of various kinds can be elicited by the proper procedure. 

 Is the frog dead? What is meant by death in the higher animals? Do all 

 parts of an animal die at the same time? What part of the frog is really 

 dead? (A.) 



B. EXTERNAL ANATOMY OF THE FROG 



Obtain an etherized frog, place it in the dissecting pan, and carefully note the 

 following points. Read also Holmes, chapter iii. The body of the animal con- 

 sists of a flattened head and a short somewhat spindle-shaped trunk. There is 



