The evidence concerning the function of these ganglionic masses is far from being even as satisfactory or conclusive as that relating to the corpora striata.
Anatomically, their relationship is equally clear; they are the ganglia of the sensory tracts, since the tegmentum or sensory parts of the crura pass directly into them. They form the chief routes by which impulses, giving rise to different sensory impressions, arrive at the cerebral cortex. But the evidence we obtain by the physiological examination of sensory impressions is indistinct compared with the results when motor tracts are excited. In the complete absence of all motion, it is impossible to know whether an animal feels or not, as we have.no other signs of the stimulus taking effect. It is difficult, as has been already seen, to stimulate any sensory tract without the impulse being reflected to its motor neighbors, so a muscular movement often results from stimulation of a group of cells purely sensory in function, and yet is not decisive evidence of conscious sensation.
When we take into consideration the foregoing points, and the fact that it is difficult, if not impossible, to destroy a portion of brain substance without irritating it and the neighboring structures, we cannot be surprised that experimenters have arrived at very contradictory results, both by stimulating and destroying the optic thalami. Some find that electric stimulation causes muscular movements; others find that it does not.
Some authorities state that destruction of the optic thalami interrupts only the incoming sensory impressions; others say it gives rise to motor paralysis.
Human pathology helps us but little, for it is impossible to say whether a lesion simply abolishes the function or acts as an irritant, or produces both these effects. Local lesions of the optic thalami have been met with, in some of which sensory, in others both sensory and motor, defects have been observed in the patients.
Fig. 257. Section through the cortex of temporal lobe of monkey, showing the series of layers of nervous cells with different characters.
We must remember that the occurrence of motion as the result of stimulation, or the absence of muscular power as the result of destruction of the optic thalami, must not be accepted as good evidence of the motor function of the nerve cells of this part, because these results may depend on the indirect influence of the sensory impulses coming from these cells.