886 A MANUAL OF PHYSIOLOGY 



3. Reflex Time. Pass a hook through the jaws. Holding the 

 frog by the hook, dip one leg into a dilute solution of sulphuric 

 acid (o'2 to o - 5 per cent.), and note with the stop-watch the interval 

 which elapses before the frog draws up its leg (Tiirck's method of 

 determining the reflex time) . Wash the acid off with waten 



Determine how the reflex time varies with the strength of the 

 stimulus. This can be done by using various strengths of acid. 

 The reflex time will be shorter the stronger the stimulus up to a 

 certain point. Compare the reflex time of movements on the same 

 side of the body as the point of application of the stimulus and on 

 the opposite side. 



4. Inhibition of the Reflexes. (i) Destroy the cerebrum of a 

 frog. Dip one leg into dilute sulphuric acid as in 3, and estimate 

 the reflex time. Then apply a crystal of common salt to the 

 upper part of the spinal cord. If the opening made for pithing 

 the frog is not large enough to enable the cord to be clearly seen, 

 enlarge it. Again dip the leg in the dilute acid. It will either not 

 be drawn up at all, or the interval will be distinctly longer than before. 



(2) Expose the viscera, including the heart, taking care not to 

 injure the cardiac nerves. Tap the intestines sharply with the 

 handle of a scalpel many times in succession. The heart is inhibited. 



(3) Tie strings tightly around both fore-legs of a normal frog. 

 Place the animal on its back ; it does not turn over. The hind- 

 legs may be pulled about in various ways without the frog turning 

 over into its normal position. The reactions concerned in the 

 maintenance of equilibrium are inhibited. Remove the strings. 

 The animal cannot be made to lie on its back except by force. 



5. Spinal Cord and Muscular Tonus. Destroy the brain of a frog. 

 Isolate the gastrocnemius, -and cut away the bone below the knee. 

 Isolate the sciatic nerve without injuring it. Remove the muscles 

 from the femur, cut the bone and fix it in a clamp for graphic 

 recording. Connect the tendon with a lever, weighted with 5 to 

 10 grammes. Take a base line. Destroy the spinal cord, or cut 

 the sciatic and again take a base line. The length of the muscle is 

 slightly altered. 



6. Spinal Cord and Tonus of the Bloodvessels. Destroy the brain 

 of a frog. Arrange the web of the foot on the stage of a micro- 

 scope, and note the calibre of the bloodvessels in the field. Destroy 

 the cord, and observe the change in their calibre. They will dilate. 



7. Action of Strychnine. Pith a frog (brain only). Inject into 

 one of the lymph-sacs three or four drops of a o - i per cent, solution 

 of strychnine. In a few minutes general spasms come on, which 

 have intermissions, but are excited by the slightest stimulus. The ex- 

 tensor muscles of the trunk and limbs overcome the flexors. Destroy 

 the spinal cord ; the spasms at once cease, and cannot again be excited. 



8. Mammalian Spinal Preparation (Sherrington).* Deeply 

 anaesthetize a cat with ether. Insert a cannula into the trachea 

 (p. 1 86), and continue the anaesthesia through this. Expose and 

 ligate both common carotids. Make a transverse incision through 

 the skin over the occiput, and extend it laterally behind the ears. 

 Pull back the skin so as to expose the neck muscles at the level of 

 the axis vertebra. Eeel for the ends of the transverse processes of 



* A similar preparation can be used for certain experiments on the 

 circulation (Crile, Guthrie). For these, as well as for the study of many 

 reflexes, a good preparation is obtained by occlusion of the cerebral blood- 

 supply in cats (without decapitation). 



