In examining the functions of the brain, we may consider the various parts in the order they are found in proceeding from the medulla toward the cerebral hemispheres. Between the medulla oblongata and the hemispheres, we come to the pons Varolii and cerebellum, the crura cerebri, and corpora quadrigemina, which, being developed from the mid-brain, may be called the mesencephalon. The duties of these parts of these nervous centres can be investigated by observing the actions of lower animals in which the hemispheres have been removed or the parts directly stimulated, and by noting the symptoms produced in man by lesions of this part of the brain. The former method gives the most definite results, and therefore deserves ' most attention.

In all these parts there are innumerable fibres capable of conducting impulses in many directions, and numerous masses of ganglionic cells distributed throughout the white substance. In the cerebellum a remarkable layer of large branching cells divides the central from the cortical tissue.

Section through a part of the cerebellum.

Fig. 255. Section through a part of the cerebellum. a, Molecular layer into which pass the branches of Purkinje's cells, b; d, Medullary centre from which medullated fibres pass through the granular layer of nervous and neuroglia cells to reach the cells of Purkinje.

When the cerebral hemispheres have been removed from a frog, the animal retains the power of carrying out coordinated motions of much greater complexity than those performed by the spinal cord alone. But this power is not exercised spontaneously. That is to say, the animal can balance itself accurately, jump, swim, swallow, etc., but it only attempts these acts when excited to do so by stimulations from its outer surroundings. Thus, on a flat surface it sits upright, but does not stir from the spot where it has been placed; if the surface upon which it sits be inclined, so that its head is too low, it turns round to regain its ordinary position. If the surface be further inclined, it at first crouches so as not to slip off, and then crawls upward to find an even resting place. Plunged into water, it swims perfectly, but on arriving in a shallow part it either rests quietly with its nose out of the water, and its toes touching the ground, or crawls out to sit on the water's edge, where it can find its balance. When touched on the leg, it jumps away from the stimulus, and in so doing avoids any obvious dark obstacle. It swallows if a substance be put in its mouth, but does not attempt to eat even if surrounded with food. In short, all movements, even the most complex, may be brought about by adequate stimulation - spontaneity only is wanting. The pupil responds by reflex contraction, when the retina is exposed to light; the eyes are closed if the light be intense; and the head may follow the motions of a flame moved from side to side. A sudden or loud noise causes it to move. From the foregoing facts, and the power such a frog has of avoiding a dark object, we may conclude that the impulses arriving from the special sense organs are all duly received, and excite more or less elaborate response, but that the consciousness of the arrival of these impulses no longer exists.

The removal of the hemispheres of birds and rabbits leaves the animal in a somewhat similar condition; but the response to the special sense impulses is not so definite or well marked, since the animal flies or runs against even the most obvious obstacles.

We may conclude, then, that the medulla controls the coordinated movements absolutely necessary for the vegetative functions, and that the mid-brain (including the cerebellum of birds and mammals) controls the complex associations of coordinated movements necessary for the perfect performance of such acts as standing and walking.

The enormous number of muscles simultaneously used in some of our commonest daily actions, concerning which we have but little thought, and take no voluntary trouble, shows the great importance of this part of the brain. If we take a simple example, that of standing in the upright position (equilibration) (see page 481), we find that a great number of muscles have to act together with the most exact nicety to accomplish what, even in man, is an unconscious, if not quite involuntary, action. In the frog, as has been seen, equilibration is performed by reflex action alone. In man the nervous mechanisms are probably more complicated by his erect attitude and the addition of the cerebellum, etc., but they are nevertheless comparable with those of the frog. It may, therefore, be instructive to examine the details of the mechanisms in a frog deprived of its cerebral hemispheres.

The optic lobes of the frog's brain (which correspond to the corpora quadrigemina, and also take the place of the cerebellum of the higher animals) form the great centres of equilibration, locomotion, etc. If these lobes be destroyed, the animal can no longer sit upright, jump or swim. The first point to determine is, whence do the impulses arrive which bring about these complex coordinations. The first set is that coming from the tactile sense of the skin of those parts touching the ground. A second set arrives from the acting muscles indicating to the centres the amount of work done {inuscular sense). A third set comes from the eyes, by which the position of the surrounding objects is gauged. Finally comes the fourth set from the semicircular canals of the internal ear, which communicate to the equilibrating centres the position of the head.

By depriving a frog of these several portals by which incoming stimuli direct the balancing centres, it can be rendered incapable of any of the acts requiring equilibration, even when the regulating centres are intact. In our own bodies we can convince ourselves of the importance of these afferent regulating impulses arriving from the eye, ear, skin and muscles. When the eyes are shut and heels together we cannot stand as steadily as when we keep our eyes fixed on something; even with care not to move, we sway slowly to and fro. If, having bent our head to the handle of a walking stick, the end of which is fixed on the ground, we run three or four times around this axis so as to disturb the fluid in the semicircular canals, and then attempt to walk straight, we find how helpless our volition becomes when deprived of the aid naturally coming from special mechanisms in the internal ear.

When the feet are "asleep" or benumbed, standing or walking can only be performed in a most awkward manner, showing the necessity of tactile sensation for perfect equilibration. In a disease known as locomotor ataxy the muscular sense is lost, and the power of standing or walking correspondingly impaired.