II. GENERAL PHYSIOLOGY OF THE FROG 



In the preceding section a general survey of the systems of organs which 

 make up the structure or morphology of an animal has been made. We may 

 next logically consider the functions or physiology of these organs, using, as 

 before, the frog merely as an example to illustrate phenomena which are common 

 to all animals. 



A. FUNCTION OF THE NERVOUS SYSTEM; IRRITABILITY, CONDUCTIVITY 



Irritability is the capacity of living matter to undergo a change as a con- 

 sequence of changes external or internal to itself. The change in the living 

 organism is known as the reaction or response, and in the case of animals the 

 response generally becomes visible as a movement. The change which produces 

 the response is the stimulus; the act of applying a stimulus to an organism is 

 stimulation. When the response appears at a different point than that to which 

 the stimulus was applied, it is quite obvious that conduction of the stimulus 

 has occurred, and this capacity of living substance to transmit stimuli is known 

 as conductivity. The time which elapses between the application of the stimulus 

 and the visible response is called the reaction time, and is evidently dependent 

 upon conductivity. The nervous system is the irritable and conductile system 

 par excellence of the body. 



Obtain from the assistant a frog pithed in the brain only (why?). Suspend 

 it by a wire through the lower jaw from the crosspieces of the electric lights. 

 Wait until it hangs quietly. Have a pan or dish of tap water handy to keep the 

 frog moist and to wash off the acid in the following experiment. Do not allow 

 the frog to become dry. Dip a very small piece of filter paper, not more than 2 mm. 

 square, into dilute acetic acid and stick it to the skin of the abdomen of the frog. 

 Response? Determine with a watch the approximate reaction time. Wash 

 off the acid with water and repeat in various ways, putting the acid on the toes, 

 skin of the hind legs, back, etc. Which part of the body is the most sensitive, 

 as determined by the reaction time? When are the fore legs used? Does the 

 reaction appear to be purposive? Under the conditions of the experiment can 

 it be so? Read Holmes, pp. 300-2. Such a reaction is called a reflex, and this 

 particular one is known as the "wiping reflex." The complete path involved in 

 such a icflex can be understood only after a more detailed study of the nervous 

 system. The steps involved are: stimulation of the sense organs in the skin 

 by the acid; conduction of the stimulation along the spinal nerves leading from 

 these sense organs to the spinal cord; conduction in the spinal cord to a level 



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