414 JOHH LINCK TJLRICH 



at the same time of its contralateral, or when the ipsilateral ex- 

 tensor thrust of the other side of the animal can not be evoked, 

 there being only one predominant reflex thrust present, then this 

 might indicate the existence of a unitary reflex rather than a 

 diphasic system of antagonistic action of flexion and extension of 

 the limbs. A diphasic system is present when there is revealed 

 symmetrical development, and a unitary mechanism when the 

 fore reflex thrusts are asymmetrically developed. A unitary 

 assymetrical development with a strong extensor tone in a right 

 or a left ipsilateral thrust must, with extensor tone in a weak fore 

 reflex thrust, be regarded for the present as an undeveloped con- 

 dition of the reflex extensor thrusts. This statement is made 

 with some reservation, for the right fore ipsilateral extensor 

 thrust is more often evoked than the left, and it will be shown 

 later that the right side of the animal invariably predominates 

 over the left. The great predominance of the right fore thrust 

 over the left may indicate that the tendency to righthandedness 

 exists or is developing in the rat. 



The reflex extensor tone of a limb may be considered weak 

 when a thrust is either incompletely, or completely but feebly, 

 evoked. Little need be said about weak extensor tone in reflex 

 extensor thrusts when these thrusts are completely evoked, other 

 than that the reflex thrust is slowly and not vigorously produced. 

 When they are incompletely evoked, the extensor thrust may in- 

 volve only the shoulder, the elbow, or the extension of the paws 

 of the fore limbs, or the knee of the hind limbs. Sometimes 

 flexion of the limbs prevails instead of extension. This is often 

 the case when it is said that all reflex thrusts are absent or can 

 not be evoked. Flexion may then be regarded as action, as well 

 as extension, but of an opposite kind. All transitional stages 

 exist, and such stages may be regarded as revealing the exist- 

 ence of a marked undeveloped functional condition, or an unequal 

 antagonistic action of extension and flexion of a rat's limbs. 



In addition to the existence of a functionally undeveloped con- 

 dition of the reflex extensor thrust, instability of them is exceed- 

 ingly common. Fluctuations in the reflex thrusts occur invari- 

 ably when one extensor thrust has a stronger extensor tone than 



