INTEGRATION OF MOVEMENTS IN LEARNING IN THE RAT 413 



When the head is passively bent to the right or to the left of 

 the midline of the body, either only one of the two ipsilateral fore 

 thrusts, or the one with a stronger extensor tone than the other, 

 is evoked. More often, in either case, a right ipsilateral fore 

 thrust prevails. At times a contralateral right or a left fore thrust 

 is synchronously evoked with an ipsilateral fore thrust; this sel- 

 dom occurs when a left ipsilateral fore thrust is evoked, but is 

 rather common in the case of a right ipsilateral fore thrust. Again 

 the right ipsilateral fore thrust is more often present than the 

 left, when the rat's head is dorsally extended 45 degrees of the 

 midline of the body. The functional condition of the reflex 

 thrusts of the hind limbs, when the position of the head is changed 

 in space, follows closely the development of the fore limb thrusts. 

 Usually when an ipsilateral fore thrust has a strong extensor 

 tone or one has a stronger extensor tone than the other, its con- 

 tralateral hind has a strong extensor tone, and when the ipsilat- 

 eral fore has a weak tone, its contralateral has a weak extensor 

 tone or lacks it altogether. When the tail is dorsally extended, 

 both hind limbs may be rigidty held backward, or they may be 

 rhythmically repeated alternately. At times, one ipsilateral 

 hind thrust is evoked with a stronger extensor tone than the other, 

 or both extensor thrusts may be absent. In some cases the reflex 

 extensor thrusts of all limbs cannot be evoked. 



A few words will not be amiss in stating what is meant when 

 it is said that an ipsilateral fore thrust has a strong or a weak 

 extensor tone or that it is absent. In this paper when it is stated 

 that extension of a limb is not followed by flexion of its contra- 

 lateral, or extension is weak, or absent, it is meant that the re- 

 flex extensor thrust or its mechanism is undeveloped. This 

 statement is however, not quite justifiable when a strong exten- 

 sor tone in an ipsilateral thrust of the fore limbs is evoked and 

 when flexion of its contralateral is not at the same time evoked, 

 or when only a strong extensor tone in one ipsilateral fore exten- 

 sor thrust can be evoked and not its contralateral. There ap- 

 pears here to be a distinct difference between the evoking syn- 

 chronously of extension and flexion of the fore limbs. When an 

 ipsilateral reflex extension of one fore limb is evoked with flexion 



