INTEGRATION OF MOVEMENTS IN LEARNING IN THE RAT 409 



thrust 1 of a limb. A single thrust was often evoked from an 

 ipsilateral fore thrust, but more often from both hind limbs. 

 Great variation in the production of a series of thrusts or of a 

 single thrust exists. 



When the method of examination of the thrusts was perfected, 

 the rat was held in the left hand in a position which permitted 

 free movement of the limbs. To evoke the extensor thrust of 

 the fore limbs, the rat's head was passively extended dorsally 

 until the lower jaw was to the midline of the body or within 45 

 degrees of this line, and also bent to the right and to the left of 

 the midline. To evoke the extensor thrusts of the fore limbs 

 when the head was dorsally extended, the right fore finger was 

 placed under the lower jaw and the head gently but firmly raised; 

 to evoke the extensor thrusts with bending the head, the muzzle 

 of the rat was seized and gently moved to the right or to the left, 

 to a point where rhythmic extention and flexion of the limbs 

 were less pronounced. The extensor thrusts of the hind limbs 

 were evoked by dorsally extending the tail and slightly raising 

 the hind quarters of the rat. 



The rat being a very docile creature when tame, no great dif- 

 ficulty was met in examining the extensor thrusts of all limbs. 

 When a rat showed excessive reflex excitability, slight trouble was 

 experienced in holding the animal in the right position to evoke 

 the extensor thrusts; the animal struggled to get away. In such 

 instances, an examination of these reflex extensor thrusts on the 

 following day gave better results. Reflexes were evoked whether 

 the head and tail were passively moved slowly or quickly. This 

 indifference in regard to the rate of movement of the head to 

 evoke the extensor thrusts is probably due to the fact that 

 changes in the position of the head arouse continuous excitations 

 in the receptors of neck muscles and in the labyrinth of the ear. 

 Passively extending the tail dorsally produced excitations in the 

 receptors of the tail and hind quarters. The effects, however, 



1 The term extensor thrust has in physiology been applied only to the exten- 

 sion reflex elicited on stimulation of the foot of a spinal animal. Proc. Roy. 

 Soc., Ixvi, p. 67, 1900. The use of this term in a wider sense, it is hoped will not 

 be confusing. 



PSYCHOBIOLOGY, VOL. II, NO. 5 



