THE FROG 8 I 



by an instructor. Kill a frog by cutting across the spinal 

 cord between the skull and the first vertebra ; then cut off the 

 entire head except the lo\ver jaw. Suspend the animal by 

 the lower jaw so that the hind limbs hang free. Pinch one 

 of the toes and note that the leg is withdrawn. The nerve 

 impulse set up by the irritation of the foot travels along 

 sensory nerve fibers (i. e., fibers which terminate in sense 

 organs) to the cord where it starts a motor impulse along 

 the nerves innervating the muscles of the leg, causing them 

 to contract. Such a direct response to the stimulus is called 

 reflex action. Touch the foot with a hot needle, or with a 

 bit of absorbent paper moistened with dilute acid, and ob- 

 serve the result. Apply dilute acid to the side of the body 

 and see if the foot is used to wipe the acid away. Cut open 

 the body cavity of the frog and lay bare the large nerves 

 going to the hind limbs. Irritate one of these nerves and ob- 

 serve the result. Cut one of these nerves across, irritate the 

 end that is in connection with the leg, and observe the effect. 



XIV. THE EYE. 



Review your observations on the external anatomy of the 

 eye of a living frog (Section II, i). The following work 

 may be done on a preserved specimen, though a freshly- 

 killed specimen is better. 



Remove an eye-ball by cutting around it through the lids, 

 and severing the muscles and the optic nerve. The muscles 

 that move the eye-ball will not be considered here, since they 

 may be studied to better advantage in a larger vertebrate 

 (e. g., an elasmobranch or a mammal). Observe how the 

 lids are connected with the eye-ball by means of a membrane 

 called the conjunctiva; clean away the fragments of the lids 

 from the eye-ball. 



