996 THE CENTRAL NERVOUS SYSTEM 



the hook, dip one leg into a dilute solution of sulphuric acid (0-2 to 

 0*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 water. 



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 microscope, 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 strych- 

 nine. In a few minutes general spasms come on, which have inter- 

 missions, but are excited by the slightest stimulus. The extensor 

 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 anaes- 

 thetize a cat with ether. Insert a cannula into the trachea (p. 202), 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. Feel for the 



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

 lation (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). Even a human 'spinal preparation' is capable of 

 executing reflex movements. The Turkomans are stated to have decapitated 

 their prisoners and immediately placed on the neck a hot metal plate, which 

 sealed up the cut vessels. The (reflex) movements, which are described as very 

 lively, were then watched with an interest, it is to be supposed, not wholly 

 scientific. 



