PRACTICAL EXERCISES 959 



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. 200), 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 

 ends of the transverse processes of the atlas, and divide the muscles 

 down to the bone just behind these processes. Now start artificial 

 respiration (p. 200), or sooner if necessary. Notch the spinous process 

 of the axis with bone forceps. Pass a strong thick ligature by a sharp- 

 ended aneurism needle close under the body of the axis, and tie it tightly 

 in the groove left by the incision behind the transverse processes of the 

 atlas and the notch made in the spinous process of the axis. This com- 

 presses the vertebral arteries where they pass from transverse process 

 of axis to transverse process of atlas. Pass a second strong ligature 



* 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. 



