ELEMENTARY EXPEEIMENTAL PHYSIOLOGY 215 



bell-jar. The destruction of these portions of the central nervous 

 system produces marked shock, but, after this has passed off, the frog 

 will still be able to jump, swim, maintain its equilibrium, and perform 

 other complicated and co-ordinated movements when it is stimulated 

 in the appropriate manner. 



The cerebellum and medulla oblongata are now destroyed by passing 

 a blanket-pin through the foramen magnum of the skull, and by 

 lateral movements of the pin breaking up the nervous tissue. The 

 frog now lies in a limp, toneless condition ; shock is well marked, 

 and does not pass off quickly. The respiratory movements of the 

 nares and of the floor of the mouth cease. The circulation of the blood 

 is disordered by the destruction of the vaso-motor centre. 



The "Spinal Animal." The frog now possesses only its spinal 

 cord, but it still shows co-ordinated movements. Its hind legs possess 

 tone, and are drawn up against the flanks ; if one leg be pulled away 

 from the body, or be stimulated by pinching a toe, it will be with- 

 drawn from the source of irritation. The movements are of a reflex 

 nature, a response to a stimulus (Fig. 203). 



When placed upon its back such a frog does not right itself, and 

 when thrown into water it generally sinks to the bottom, and may or 

 may not swim for one or two strokes. 



If such a frog be suspended by the lower jaw, it does not move 

 unless stimulated. 



A small piece of filter-paper soaked in strong acetic acid will if 

 placed upon the skin of one flank, act as a 

 stimulus, and the leg of the corresponding side 

 will be raised to wipe off the offending body. If 

 this experiment be repeated five minutes after 

 the frog has been dipped in a beaker of water 

 to remove the acid, and the leg be held down 

 by the hand, then the leg of the opposite side 

 will be raised in an apparent endeavour to wipe 

 off the irritating piece of paper. The frog is 

 again dipped in the beaker of water to remove 

 the acid. 



Turck's experiment upon the time of response I 

 of the spinal animal to a stimulus can now be Fia 204 - ~ A metronome - 

 performed. A small beaker is filled with dilute sulphuric acid (1 in 

 1000), and is gradually raised until the toes of one of the hind legs 

 dip into the acid ; this moment is noted, and then the interval between 

 the application of the acid and the withdrawal of the toes is measured 

 by a watch or a metronome (Fig. 204). After washing off the acid the 



