60 LABORATORY MANUAL OF GENERAL PHYSIOLOGY 



column. Arrange 2 pairs of electrodes (rocking key) to stimulate the 

 nerves on each side of the fragment of chord. Record a twitch in each case 

 with rapid drum and tuning fork. Compare latent periods. For prop- 

 erties of the synapse see Bayliss, p. 141; Eccles, Er. Physiol., 38: 339, 1936. 



3. Summation in a Reflex. — Pith the brain but do not pith the spinal 

 cord of a frog and suspend the animal by a hook in the jaw. Tie two fine 

 copper wires 1 cm. apart around the toes of the left foot. Connect wires 

 with secondary coil of inductorium. Connect primary with single cell 

 and simple key. Can reflex action result from summation of weak make 

 and break shocks? Try a tetanizing current of subliminal stimuli (cf. 

 Bayliss, p. 489). 



4. Inhibition of a Reflex. — Immerse toes of right leg in weak acetic 

 acid solution and note time for the reflex. Wash off the acid. Now 

 stimulate the left foot with weak tetanizing current as the right is again 

 immersed in the acid. Result? (Bayliss, p. 410.) 



5. Chemical Stimulation by Ions. — Dip the frog's foot into 2 M LiCl 

 and time the latent period of reaction. Compare with 2 M KCl. The 

 stimulating efficiency of ions is a function of their mobility determined 

 by molecular weight and their shell of water molecules. Li, although 

 of small mol. wt. (7), moves more slowly than K (39) because of hydra- 

 tion. Li carries 150 water molecules, K only 20. Hopkins (J. Exp. 

 Zool., 61: 13, 1932) discusses ionic mobility and stimulation (cf. 

 Maass and Steacie, Physical Chemistry, 1931, p. 220, for ionic movement 

 and hydration). Text p. 34. 



6. Irradiation of Reflexes. — Gradually increase the strength of a very 

 weak tetanizing current and note effect on the reflex movement of the 

 various limbs. (Crossed reflexes.) 



7. Reflex Tonus. — Note position of the legs of the suspended frog. 

 Slit abdomen and cut roots of all nerves going to right leg where they 

 leave the spinal cord. Again suspend the frog and note position of the 

 legs (cf. von Buddenbrock, "Grundriss der vergl. Physiol.," p. 186). 



CENTRAL NERVOUS SYSTEM OF CRAYFISH 



Directions from "Laboratory Outlines in Comparative Physiology," 

 by C. G. Rogers, McGraw-Hill, 1929. 



8. Functions of Circimioesophageal Commissures. — Carefully expose 

 the circumoesophageal commissure on the right side of a crayfish (Fig. 

 5). Insert probe in mouth and dissect from one side. Stimulate with 

 a weak tetanizing current and observe effect on body and limbs. Note 

 effect on voluntary movement of appendages. Note extension with strong 

 currents (reverse of peripheral nerve stimulation see p. 54). Cut left 

 circumoesophageal commissure. Does the animal now make spontaneous 



