touched motions of the eyes or head occur in the plane of this canal. In this case we have to deal with a simple relation between the orientation of the canal and the plane in which the organs or the whole body of the animal move. This fact is just as mysterious as the more general facts mentioned in this chapter. I am inclined to assume that the peculiar relation between the semicircular horizontal canal and the motions produced by the stimulation of this canal finds its explanation through the facts mentioned in this chapter. It is possible that the central endings of the nerve of the horizontal canal are connected with the motor elements in the medulla whose activity produces motions in the plane of the horizontal canal. When Flourens made his experiments on the semicircular canals he found that there was a striking resemblance between the effects of a destruction of the canals and the sectioning of the crura cerebelli. He came to the conclusion that there must be a simple relation between the direction of the fibres of the crura cerebelli and the motion produced by them (3). His observations are not in all points correct; yet with some modification his fundamental idea remains true.

The next chapter, on the cerebellum, will give us some more data about his observations.

It is possible for us to conceive from this how it happens that the same optic stimulus or the same space-perception is able to direct our eyes toward a certain point, to turn our head in that direction, to guide our finger thither, or to bring our legs into such

activity that our body arrives at that place. It is possible that the elements of the central nervous system which become active in this way all have the same orientation in each segment, and what we call an innervation may be a process in which the orientation of the elements plays a rôle. The effect of the electric current might be an example of such a process. This problem of the physiology of coördinated movement which we touch upon here has always seemed to me the most mysterious in the whole physiology of the central nervous system, and the way offered here of reaching a simple solution seems to me worthy of mention. The whole conception can easily be classified under the segmental conception of the central nervous system. Movements of the eyes, head, arms, and legs depend upon as many different segmental ganglia. Each of these ganglia has some features in common with every other ganglion, for instance the orientation and arrangement of the elements (neurons?). If a process of such a nature that it can only stimulate elements oriented in a certain way in each ganglion spreads through the segmental ganglia, it must produce a movement in exactly the same direction in the appendages of each segment. This does This does away with the necessity of imagining artificial connections of the neurons which would be able to produce such a series of coördinated motions in different limbs and segments.

If the question be raised, however; as to how it happens that a simple relation exists between the orientation of the motor nerve-elements and the

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movement or progressive movement produced by them, we must again refer to the simple segmental relations of the first embryonic formation that remains better preserved in the central nervous system than in the muscles. The problem with which we have to deal here is ultimately a problem of embryology.

BIBLIOGRAPHY.

Zur Theorie des Gal

1. LOEB, J., and WALTER E. GARREY. vanotropismus. II. Mittheilung. Versuche an Wirbelthieren. Pflüger's Archiv, Bd. lxv., 1896.

2. LOEB, J., and S. S. MAXWELL. Zur Theorie des Galvanotropismus. Pflüger's Archiv, Bd. lxiii., 1896.

3. FLOURENS, P. Fonctions du Système nerveux. Paris, 1842.

CHAPTER XII

EXPERIMENTS ON THE CEREBELLUM

The experiments on the cerebellum support to a certain extent the observations mentioned in the preceding chapter.

The cerebellum, like the cerebral hemispheres, is a structure which clearly expresses inequality of growth. Both may be considered as evaginations and appendages of the segmental nervous system. The cerebellum is connected with the central nervous system by three crura, the crura cerebelli ad medullam oblongatam, the crura cerebelli ad pontem, and the crura cerebelli ad corpora quadrigemina. The latter extend forward in a pretty straight line, the first extend backward, and the peduncles to the pons at right angles to both. Magendie discovered and Flourens confirmed the fact that lesion of these tracts possessing so characteristic an orientation to the chief axes of the body produces "forced" movements whose direction bears a simple relation to the orientation of the severed peduncle. If a peduncle of the pons be severed on one side the animal rolls about its longitudinal axis. If the crura cerebelli that extend forward be severed the animal rushes forward with

great force; if the crura cerebelli ad medullam oblongatam be severed the animal goes backwards or shows a tendency to turn somersaults backwards. “La direction des mouvements produits par la section des fibres de l'encéphale est donc toujours determinée par la direction de ces fibres" (Flourens).

Flourens called attention to the analogy of these phenomena with those he observed after lesion of the semicircular canals. This analogy, however, does not exist just as he states it. He compares the effect of the one-sided division of the pons with the division of a horizontal canal. This is not correct. So far as I know, such a lesion does not produce rolling motions about the longitudinal axis in any animal. On the other hand, destruction of a whole ear, probably in most cases, causes rolling motions. Flourens states further that after destruction of the anterior canals an animal turns somersaults forwards, after destruction of the posterior canals backwards. Flourens assumes that the nerves of the three canals continue into the corresponding peduncles of the cerebellum, and that this origin of the nerves is the cause of the phenomena that we observe after lesion of the single semicircular canals (3). But this is probably not correct, since the auditory nerve ends in the medulla. It is possible, however, that the cerebellum is connected with the same motor elements in the medulla with which the acoustic nerve is connected. The cerebellum might thus appear as an appendage of the acoustic segments.