CHAPTER XXVIII. 

 DISSECTION OF THE FROG. 



Pithing 1 . Stun the frog by a sudden blow on the head with the 

 stick provided. Pass the blunt edge of the knife over the dorsal surface 

 of the head until the groove at the back of the occiput is felt. At this 

 point and in the middle line plunge the knife transversely into the 

 vertebral canal, so as to sever the cord (Fig. 29, 1). 



Make the incision as narrow as possible, so as to miss the vertebral 

 arteries, the division of which will cause much bleeding. 



Pass a metal seeker upwards into the cranial cavity and turn it 

 round freely, so as to disorganise the brain completely. Decerebration. 



To complete the pithing, pass the seeker down the full length of the 

 canal, so as to destroy the spinal cord. 



The hind limbs will twitch violently whilst this is being done, but 

 no sensation can be set up, as the brain has been destroyed. If bleeding 

 is to be prevented, plug the aperture in both directions with pointed 

 match ends. 



Dorsal dissection. Lay the frog on the loaded frog plate, dorsal 

 surface uppermost and with its legs extended. 



Find the longitudinal groove in the skin in the middle of the 

 thigh, which marks the position of the biceps muscle. Under this 

 muscle lies the sciatic nerve. 



Pinch up the skin with forceps, make an incision lengthways up 

 the middle of the thigh to the illiac bone and along the whole of its 

 inner edge. Use scissors whenever you can. 



Next prolong the incision downwards to the middle of the foot. 

 Pin the skin aside after severing the few attachments at thigh, knee, 

 and heel. 



Note the dorsal cutaneous nerves as the skin of the back is 

 reflected. The skin has few adhesions to the subjacent parts, as 

 extensive lymph sacs intervene. 



Clear the narrow biceps carefully from its neighbours with scissors, 

 the nerve will be found beneath it, crossed from without inwards by 

 the sciatic artery. 



