The large size and complicated structure of this organ in the higher vertebrate animals, and its distinctness from the cerebrum,—for its commissural connection with that segment of the encephalon is not extensive,—have excited the interest and curiosity of speculative physio logists ; and, accordingly, we find no part respecting which a greater variety of hypotheses have been suggested, most of them being en tirely devoid of foundation. The experiments of Flourens have, however, thrown more light on this subject than any previous observations; and his hypothesis appears nearer the truth than any which has been proposed.
The facility with which the cerebellum may be removed or injured, especially in birds, without involving the other segments of the bmin, renders it a much more favourable ob ject for direct experiment than them. A skil ful operator may remove the greater part or the whole of the cerebellum without inflicting any injury on the hemispheres or other parts.
Flourens removed the cerebellum from pi geons by successive slices. During the removal of the superficial layers there appeared only a slight feebleness and want of harmony in die movements, without any expression of pat?. On readiing the middle layers an almost uni versal agitation was manifested, without nuy sign of convulsion : the animal performed rapid and ill-regulated movements; it could hear and see. After the removal of the deepest layers, the animal lost completely the power of stand ing, walking, leaping, or flying. The power had been injured by the previous mutilations, but now it was completely gone. When placed upon his back, he was unable to rise. Iie did not, however, remain quiet and motionless, as pigeons deprived of the cerebral hemispheres do ; but evinced an incessant restlessness, and an inability to accomplish any regular or defi nite movement. He could see the instrument raised to threaten him with a blow, and would make a thousand contortions to avoid it, but did not escape. Volition and sensation re mained ; the power .of executing movements remained; but that of coordinating those inove men ts into regular and combined actions was lost.
Animals deprived of the cerebellum are in a condition very similar to that of a drunken man, so far as relates to their power of locomotion. They are unable to produce that combination of action in different sets of muscles which is necessary to enable them to assume or main tain any attitudes. They cannot stand still for a moment; and, in attempting to walk, their gait is unsteady, they totter from side to side, and their progress is interrupted by frequent falls. The fruitless attempts which they make
to stand or walk are suffictent proof that a cer tain degree of intelligence remains, and that voluntary power continues to be enjoyed.
Rolando had, previously to Flourens, ob served effects of a similar nature consequent upon mutilation of the cerebellum. In none of his experiments was sensibility affected. The animal could see, but was unable to exe cute any of the movements necessary for loco motion.
Flourens' experiments have been confirmed by those of Hertwig in every particular, and they have been Lately repeated with similar re sults by Budge and by Longet. The removal of part of the cerebellum appears capable of pro ducing the same vertiginous affection which bas been already noticed in the case of deep injuries to the mesocephale. After the well known experiments of Magendie, of dividing either crus cerebelli, the animal was seen to roll over on its long axis towards the side on which the injury was inflicted.
The effects of injuries to the cerebellum, ac cording to the reports of the experimenters above referred to, contrast in a very striking manner with those of the much more severe operation of removing the cerebral hemispheres. " Take two pigeons," says M. Longet ; " from one remove completely the cerebral lobes, and from the other only half the cerebellum ; the next day, the first will be firm upon his feet, the second will exhibit the unsteady and un certain gait of drunkenness." Experiment, then, appears strikingly to fa vour the conclusion which Flourens has drawn, namely, that the cerebellum possesses the power of coordinating the voluntary movements which originate in other parts of the cerebro-spinal centre, whether these movements have reference to locomotion or to other objects.
That this power is mental, i. e. dependent on a mental operation for its excitation and ex ereise, is rendered probable from the experience of our own sensations, and from the fact that the perfection of it requires practice. The vo luntary movements of a new-horn infant, al though perfectly controllable by the will, are far from being coordinate they are, on the contmry, remarkable for their vagueness and want of definition. Yet all the parts of the cerebro-spinal centre are well developed, except the cerebellum and the convolutions of the ce rebrum. Now, the power of coordination im proves earlier and more rapidly than the intel lectual faculties; and we find, in accordance with Floulens' theory, that the cerebellum reaches its perfect developetnent of form and structure at a much earlier period than the hemispheres of the cerebrum.