ELEMENTARY EXPERIMENTAL PHYSIOLOGY 



15 



causes a discharge of motor impulses to the muscles of the body, which 

 give a series of convulsive twitches or contractions. These twitches 

 quickly cease, the body and limbs are in a toneless, relaxed condition, 

 and all reflexes have been abolished. 



The frog is then placed belly downwards on a frog-board, and the 

 skin at the ankle is divided by a circular incision ; the tendo-Achillis is 

 exposed and a thread passed under the tendon and tied just above the 

 sesamoid bone. In this way a ligature is attached to the muscle with- 



Fio. 20. FIG. 21. 



Muscles of the frog's leg. (After Ecker.) 



FIG. 20. Dorsal aspect. 



1. Trinceps femoris. 



2. Biceps femoris. 



3. Rectus interims. 



4. Semi-membranosus. 



5. Gastrocnenrius. 



6. Tendo Achillis. 



FIG. 21. Ventral aspect. 



1. Rectus internus. 



2. Gracilis. 



3. Adductor longus. 



4. Vastus internus. 



5. Sartorius. 



6. Adductor brevis. 



7. Adductor magnus. 



8. Gastrocnemius. 



9. Tendo Achillis. 



out damage to or irritation of its fibres. The tendon is divided below 

 the sesamoid bone, and a pull upwards towards the knee frees the 

 gastrocuemius muscle and the skin from the remaining structures of 

 the leg, which are cut away just below the knee. The gastrocnemius 

 muscle is protected from drying and from contact with foreign sub- 

 stances by drawing down the "trouser" of skin. The sciatic nerve 

 is now dissected in the following way. The skin over the posterior 

 surface of the thigh is divided by a longitudinal incision in the middle 

 line, the biceps and semi-membranosus muscles are separated, and the 

 sciatic nerve is exposed. The nerve must not be pinched with forceps, 

 for it is easily damaged. The muscles on each side of the urostyle and 

 then the urostyle itself are cut away ; the three constituent ends of 

 the sciatic nerve are now exposed. The spinal column is divided 

 transversely between the 6th and 7th vertebrae and the 9th, 8th, and 

 7th vertebrae are bisected. The piece of bone, from which the nerve 

 to be prepared issues, can be grasped with the forceps without damage 



