248 EXPERIMENTAL PHYSIOLOGY. 



(which is particularly evident in young animals) and marked by a cut with a 

 scalpel. The preparation is now ready for decerebration. This is best done by 

 using a planing blade (12.5 cm. wide, 9 cm. high and 4 cm. thick) bevelled on one 

 edge and mounted on a wooden handle, but a good ax of about the same dimen- 

 sions is quite satisfactory. To perform the decerebration, the operator holding 

 the blade in the left hand, places the edge on the notched mark, and holds the 

 plane of the blade at right angles to the plane of the head. By means of a heavy 

 mallet (or hammer, if an ax is used) held in the right hand he gives the blade a 

 light blow so as to make the edge engage the skull, the blade being held accurately 

 transverse to the long axis of the skull and directed so that when the head is cut 

 through the edge will strike a mark made on the metal plate of the base of the 

 tongue guard. The operation is then completed by applying a couple of strong 

 blows and the decerebrated animal is immediately removed from the stand and 

 placed on its back on the table, the neck being grasped with the finger and thumb 

 just below the transverse process of the atlas so as to compress the vertebral 

 arteries. The head is kept elevated to diminish haemorrhage, and etherization is 

 discontinued. In many animals the respirations cease for a few moments, and 

 artificial respiration may be necessary. They usually return spontaneously, but 

 if they do not do so, the vertebral arteries should be momentarily released so as to 

 allow some blood to flow to the respiratory centre. The bleeding from the cut 

 across the head is not usually serious if the vertebrals are properly compressed 

 and when time has been allowed for clotting to occur these may be cautiously 

 released. There is no particular advantage in trying to accelerate the clotting 

 by applying absorbent cotton wool to the wound. It usually takes from 15 to 20 

 minutes before the vertebrals can be entirely released. Finally, the clamps are 

 removed from the carotid arteries, one at a time any bleeding point in the cut 

 muscles being ligated and the head is tied to an upright so as to keep it elevated. 



The resulting preparation is suitable for many experimental 

 purposes (consult Sherrington's Mammalian Physiology, The 

 Clarendon Press, Oxford.) 



The blood pressure, although it may rise considerably while 

 the vertebrals are being compressed (because of partial asphyxia) 

 soon returns to about the normal level, and the respiratory centre 

 responds to changes in C H of the blood much more promptly than 

 in anaesthetised animals. Why should this be the case? It is 

 particularly in the study of reflex actions, however, that the pre- 

 paration is of value, although certain of them are masked by the 

 decerebrate rigidity which develops. Since the section if properly 

 made goes through the mesencephalon just behind the anterior 

 corpora quadrigemina, reflexes involving the head end of the animal 

 can be demonstrated. The following are to be elicited and studied : 

 1. THE PINNA REFLEX. Pinch or slightly twist the tip of the 



pinna between the finger and thumb. The response consists of 



