Every student learns as established facts :-
(1) That the pre-Rolandic "motor" area of the cerebral cortex is the active seat of the will in the performance of voluntary actions. Further, that the axons of the pyramidal cells of this area ("upper motor neurones") pass along the brain-stem, cross either via, the decussation of the pyramids or lower in the cord to end in terminal arborizations around the "second" or "lower motor neurones"
in fhe ventral grey column of the spinal cord. The axons of these cells pass out in the ventral nerve roots to end as "motor" fibres in voluntary muscles.
(2) That in the cerebral cortex are represented not the actions of individual muscles, but "pictured movements." (F. Wood Jones : Principles of Anatomy as Seen in the Hand. London: 1920.)
(3) That the cerebellum exercises a synergic control over all the muscles taking part in such "pictured movements." This control is under the guidance of the cerebrum. Thus at the same time that an impulse is sent along the
"upper motor neurone"—the "private path" of the cerebral cortex (Sherring- ton)—to a ventral horn cell of the opposite side, an impulse is sent from the motor cortex, decussating in the pons via the cerebro-ponto-cerebellar fibres to the cerebellum of the opposite side (1st and 2nd neurones). From the cerebellar cortex it is carried by the 3rd neurone to the nucleus dentatus ; thence by the decussation of the brachia conjunctiva to the red nucleus of the opposite side, i.e., the side from which it started (4th neurone) ; thence by the rubro-spinal tract and the decussation of Forel to the opposite side again, and thence to the ventral horn cells (5th neurone). This constitutes the "private path" for the cerebellar synergic control of pictured movements. Other "private paths," viz., tecto-spinal, vestibulo-spinal, olivo-spinal, and certainly others, influence the ventral horn cell or "lower motor neurone," whose axon or peripheral nerve- fibre constitutes a "public" or "common pathway" for all these influences.
The point to be stressed here is that, coincident with the passage of the direct impulse from the cerebral cortex to the muscles involved in a voluntary movement, another impulse, which apportions the exact degree of contraction and relaxation to each muscle involved, travels from the cerebellum, started, as it were, by a telephone call from the cerebrum.
148 THE SPECULUM.
The pons Varolii (pars basilaris pontis) forms the great pathway for the cerebro-ponto-cerebellar fibres. It is developed only in mammals. Lateral lobes or hemispheres of the cerebellum, when present in other vertebrates, are only rudimentary ; in mammals alone do they reach any high degree of development.
An inspection of the models in the Anatomy Museum shews that the development of the pons and of the lateral lobes of the cerebellum march hand in hand, and that while both these structures are developed in varying degrees in all mammals they are both particularly well developed in the anthropoid apes and man.
Wood Jones (Arboreal Man, London : 1916) has come to the conclusion that the earliest mammalian stock took to an arboreal life before the forelimb had ever been stabilized for the purposes of quadrupedal support. Animals which adopted a quadrupedal pronograde mode of terrestrial life sacrificed the mobility of the forelimbs for a gain in stability. With the loss of mobility were lost also the many methods of becoming much more fully acquainted with their immediate environment, which are possible by the use of a mobile, though primitive, fore- limb and hand. The relatively immense neopallium of man and the anthropoid apes he ascribes to the opportunities given by this mobile forelimb and hand of gaining a knowledge of the environment far superior to that which could be gained by the use of the most delicate snout, which is practically the only organ that is available to the quadruped for the conveyance of impressions of tactile discrimination to the brain.
This is not intended to be a recapitulation of the arguments used in support of this theory by the author of Arboreal Man. These could not be put more plainly or convincingly than they are in that work, and any attempt at recapitula- tion is therefore unnecessary. I propose to take as a basis of argument the premise that the cerebral cortex of man owes its degree of development to the manifold activities of an emancipated forelimb, and I suggest further that the lateral lobes of the cerebellum and the pars basilaris pontis owe their develop- ment to the same agency.
Professor Berry says : "The degree of development of the pons varolii (pars basilaris pontis) is significant as an index of the degree of skilled movements of which the animal is capable."
Recent work on the localization of function in the cerebellar cortex shews that almost the whole of the surface of the lateral lobes is concerned with the upper extremity, and particularly the hand. A much smaller area subserves the needs of the lower extremity, while the relatively small median lobe or vermis is concerned with the trunk and head (Ingvar).
These two statements, taken together, surely indicate that the course of evolution of these structures has been that outlined above, for in no other way can we so readily explain the coincidence of the large area devoted to the hand with the degree of skilled movements which is in turn correlated with the develop- ment of the pars basilaris pontis (Berry, vide supra), for the greater part of our skilled movements are performed with our hands. We must remember in con- sidering the evolution of any part of the nervous system that it was actually built by the inflowing sensations and impressions gained from the animal's various contacts with its environment. Such a statement as this last, it must be noted, does not necessarily imply a belief in the inheritance of "acquired characters" or individual modifications of the soma in response to change of function. It is quite consistent with Weismann's "germinal selection," and with the more recent work of the Mendelians or mutationists.
The amount of synergic muscular action involved in a delicate skilled move- ment is much greater than is generally recognized. If we think, however, of the degree of muscular exhaustion that follows the performance of so common an action as threading a fine needle, we cannot help realizing that a great deal more muscular effort has been expended than is evident from the amount of work done. The streams of nervous impulses which are presumably, during the per- formance of such a skilled movement, flowing out from the cerebellum, via the rubro-spinal tract and the lower motor neurone must involve a heavy drain on the nervous energy of the cerebellum. This is in accord with Professor Berry's view that the cerebellar cortex acts as a storehouse for nervous energy.
Although the maintenance of bodily equilibrium cannot be considered entirely apart from the synergic control of skilled movements, since both these functions are concerned with the position in space of the parts of the body involved, yet it must be evident that the maintenance of equilibrium of the body as a whole is a far more primitive function than the other. It is therefore consistent with modern evolutionary ideas to expect the more primitive structure, the vermis, to be concerned primarily with the more primitive function, and the newer portion, the hemispheres, to be concerned primarily with the newer function. That the hemispheres or lateral lobes can be concerned only secondarily with bodily equilibration is apparent when we consider—
(1 ) The brain of a bird ; (2) The brain of a fish ; (3) The brain of a bat.
Bodily equilibration is supremely necessary to these three classes of animals.
In the bird the median lobe or vermis is large and prominent, the lateral lobes are rudimentary, and the pars basilaris pontis is absent. Yet the bodily equilibra- tion of the bird is probably more nearly perfect than that of any other animal.
It is evident that in the bird the site of this function cannot be the lateral lobes of the cerebellum.
Exactly the same reasoning applies to the fish. Neither of these animals can perform individual skilled movements, although there is a perfect reflex mechanism in the vermis for the highly accurate generic movements of flying and swimming.
The bat, which needs perfect equilibration, possesses the common mammalian heritage of a pons and lateral cerebellar lobes, yet these structures are no more developed in the Cheiroptera than in the Marsupials and the Insectivora, and.
indeed, not so well developed as in the Rodentia (Owen, Anat. and Physiol. of Vertebrates. Vol. III.. pp. 89-90), whose need for perfection of equilibration may he assumed to be less than that of the bats. The function of bodily equilibration would not then appear to have its site in the lateral lobes, an inference which agrees with the localization of function as hitherto determined in the cerebellar cortex of man and the anthropoid apes.
According to Owen (loc. cit.), "the Cetacean brain is remarkable for the large size of the cerebellum, and especially of its lateral lobes, . . . the pons is now large and prominent." At first sight this appears to constitute a direct contradiction to the conclusions outlined above, for no marine mammal can be considered capable of performing skilled movements with its forelimbs. What we should expect to find is a condition somewhat similar to that of the Cheirop- tera, for both stocks possessed the common mammalian heritage of a cerebro- ponto-cerebellar pathway, and both departed from a terrestrial habitat to life in a medium which demanded specialization in bodily equilibration. However, it
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150 THE SPECULUM.
is not stretching the probabilities of the case too far to suppose that there should be some distinct differences in adaptations to life in aerial and marine media respectively. What Owen took for lateral lobes may still be truly called lateral lobes in the sense that they are lobes and lateral in position, but on the authority of Elliot Smith we may conclude that morphologically they are of different origins. The lateral lobes of the Cetacea are the homologues of the human para- flocculi or flocculi secundarii, which in man are merely a pair of rudimentary buds. Elliot Smith (Cat. Royal Coll. Surgeons, 2nd Edt., Vol. 2, 1902) describes the paraflocculi as forming the whole of the lateral aspects of the cerebellum in the manatee and the dugong of the Order Sirenia, and figures a similar arrangement in Cetacean brains. In Cunningham's Text-book of Anatomy (5th Edit., p. 571) the same author says : "The enormous development of the paraflocculus in all marine mammals is due to the fact that it co-ordinates these muscular movements that maintain equilibrium by controlling the tendency to roll in the lateral direction, that is, around the long axis of the body," and further that "the evidence of comparative anatomy seems to suggest that the reduction of the paraflocculus in man and the apes is associated with the adoption of the erect attitude (or in the case of the apes the maintenance of equilibrium by the use of the arms)." We might go even further than this last statement by considering that since man's "assumption of the erect posture" is merely a further step in the same direction, resulting from a similar evolution in the same arboreal habitat as that of the apes, the reduction of the pa.raflocculus in both stocks is due to the same process. Incidentally it is worth noting that in all the Mammalia except the Anthropoidea, that is, in all the stocks which, on Wood Jones' hypothesis, did not adopt primitively an arboreal habitat, the paraflocculus attains considerable dimensions. (Elliot Smith, Journ. of Anat. and Physiol., Vol. 37, 1903, and Cat. Royal Coll. Surg.).
That the pons may not have been the only pathway that shared in the co-ordi- nate development of the cerebral cortex and the cerebellar hemispheres is shewn by the following quotations, also from Elliot Smith's article in Cunningham :
"There seems to be a direct relationship between the size of the inferior olivary nucleus and the extent of the cortical area that presides over highly skilled movements." We know that the bulk of the restiform body is made up of olivo- cerebellar fibres, and these fibres "convey impulses from the higher regions of the brain, directly or indirectly (probably the latter), from the motor area of the cerebral cortex." Apparently the inferior olivary nucleus may be looked upon as an adjunct to the nuclei pontis, and its development may have a similar evolutionary history to the basilar part of the pons.
Conclusion.—Assuming, then, that the relatively immense development of the human neopallium is due to the opportunities for education given by an emancipated forelimb, the by-product of the primitive assumption of an arboreal habitat, we may conclude that the development of the cerebellar hemispheres, the pars basilaris pontis, and possibly the inferior olivary nuclei, marched with equal pace, and that the huge size of the cerebellar hemispheres is an expression of the need for synergic and postural control of a forelimb which the exigencies of arboreal life made the chief and in the animal world by far the most efficient building agent of conscious will as expressed in that vast collection of neurones and association-paths which we name the cerebral cortex.
In all the mammals except the Anthropoidea the main control of the organism is the automatic and more or less unconscious thalamo-striate brain ;
the cerebral cortex is relatively speaking rudimentary, and the cerebellum con- sists of a large central lobe, small lateral lobes, and relatively large paraflocculi.
As quoted above, the paraflocculi are considered to be centres for muscular con- trol for prevention of sideways rolling around the long axis of the body. In the pronograde or quadrupedal mammals, where such a tendency exists to some degree, the paraflocculi are large ; in marine mammals they are immense. In an arboreal animal this tendency is at its minimum, hence the shrinkage, or, rather, the non-development, of the organ in the Anthropoidea. Hence also the develop- ment from a morphologically distinct area of the cerebellum of an organ to meet the special necessities of arboreal life, where the hind-limbs only are stabilized for locomotion, and the fore limbs are left free—emancipated—to grasp, to judge, and perhaps even to balance.
—F. R. GUINANE.