what complicated. They are still more so in birds and quadrupeds; and finally, in the human subject they become both varied and complicated in the highest degree. Even in maintaining the ordinary postures of standing and sitting, there are many different muscles acting together, in each of which the degree of contraction, in order to preserve the balance of the body, must be accurately proportioned to that of the others. In the motions of walking and running, or in the still more delicate movements of the hands and fingers, this harmony of muscular action becomes still more evident, and is seen also to be absolutely indispensable to the efficiency of the muscular apparatus.

The opinion which locates the above harmonizing or associating power in the cerebellum was first suggested by the effects observed after experimentally injuring or destroying this part of the brain. If the cerebellum be exposed in a living pigeon, and a portion of its substance removed, the animal exhibits at once a peculiar uncertainty in his gait, and in the movement of his wings. If the injury be more extensive, he loses altogether the power of flight, and can walk, or even stand, only with great difficulty. This is not owing to any actual paralysis, for the movements of the limbs are exceedingly rapid and energetic; but is due to a peculiar want of control over the muscular contractions, precisely similar to that which is seen in a man in a state of intoxication. The movements of the legs and wings, though forcible and rapid, are confused and blundering; so that the animal cannot direct his steps to any particular spot, nor support himself in the air by flight. He reels and tumbles, but can neither walk nor fly.

The senses and intelligence at the same time are unimpaired. It is extremely curious, as first remarked by Longet, to compare the different phenomena produced by removal of the cerebrum and by that of the cerebellum. If we do these operations upon two dif ferent pigeons, and place the animals side by side, it will be seen that the first pigeon, from whom the cerebrum only has been removed, remains standing firmly upon his feet, in a condition of complete repose; and that when aroused and compelled to stir, he moves sluggishly and unwillingly, but acts otherwise in a perfectly natural manner. The second pigeon, on the other hand, from whom the cerebellum only has been taken away, is in a constant state of agitation. He is easily terrified, and endeavors, frequently with violent struggles, to escape the notice of those who are watching him; but his movements are sprawling and unnatural,

and are evidently no longer under the effectual control of the will. (Fig. 142.) If the entire cerebellum be destroyed, the animal is no longer capable of assuming or retaining any natural posture. His legs and wings are almost constantly agitated with ineffectual

struggles, which are evidently voluntary in character, but are at the same time altogether irregular and confused. Death generally takes place after this operation within twenty-four hours.

The results of the above experiment are extremely constant and invariable, and by themselves would lead us to adopt, with a good degree of confidence, the opinion of Flourens. This opinion evidently has more direct evidence in its favor than any other theory which has yet been broached with regard to the function of the cerebellum. Many facts derived from comparative anatomy tend, also, to confirm the same opinion. If we compare different classes of animals with each other, as fish with reptiles, or birds with quadrupeds, in which the development and activity of the entire nervous system vary extremely, the results of the comparison will be often contradictory; but if we compare different species belonging to the same class and order, in which the general structure and plan of organization are nearly the same, we often find the development of the cerebellum to correspond very closely with the perfection and variety of the muscular movements. The frog, for example, is an aquatic reptile provided with anterior and posterior extremities; but its movements, though rapid and vigorous, are exceedingly simple in character, consisting of little else than flexion and

extension of the posterior limbs. The cerebellum in this animal is exceedingly small, compared with the rest of the brain; being nothing more than a thin, narrow ribbon of nervous matter, stretched across the upper part of the fourth ventricle. In the common turtle we have another aquatic reptile, where the movements of swimming, diving, progression, &c., are accomplished by the consentaneous action of both anterior and posterior extremities, and where the motions of the head and neck are also much more varied than in the frog. In this instance the cerebellum is very much more highly developed than in the former. In the alligator, again, a reptile whose motions, both of the head, limbs, and tail, approach very closely to those of the quadrupeds, the cerebellum is still larger in proportion to the remaining ganglia of the encephalon.

In the above instances, therefore, an evident correspondence exists between the size of the cerebellum and the variety of movement of which the animal is capable. Still, this part of the subject has not yet been sufficiently investigated to enable us to say that such a correspondence exists in all cases. Morbid alterations of the cerebellum, furthermore, such as inflammations, abscess, tumors, &c., have not always been found to produce, in the human subject, symptoms connected with a loss of harmony in the voluntary movements. The complete function of the cerebellum, accordingly, cannot yet be regarded as positively ascertained; but so far as we may rely on the results of direct experiment, and on the general facts of comparative anatomy, the most plausible opinion is that of Flourens, viz., that the cerebellum possesses the power of uniting and harmonizing the action of separate muscles, so that they may assist each other in the production of varied and complicated movements.

TUBERCULA QUADRIGEMINA.

These bodies, notwithstanding their small size, are very important in regard to their function. They give origin to the optic nerves, and preside, as ganglia, over the sense of sight; on which account they are also known by the name of the "optic ganglia." Their development corresponds very closely with that of the external organs of vision. Thus, they are large in fish, reptiles, and birds, in which the eyeball is for the most part very large in proportion

to the entire head; and are small in quadrupeds and in man, where the eyeball is, comparatively speaking, of insignificant size. Direct experiment also shows the close connection between the tubercula quadrigemina and the sense of sight. Section of the optic nerve at any point between the retina and the tubercles, produces complete blindness; and destruction of the tubercles themselves has the same effect. But if the division be made between the tubercles and the cerebrum, or if the cerebrum itself be taken away while the tubercles are left untouched, vision, as we have already seen, still remains. It is the tubercles, therefore, in which the impression of light is perceived. So long as these ganglia are uninjured and retain their connection with the eye, vision remains. As soon as this connection is cut off, or the ganglia themselves are injured, the power of vision is destroyed.

The tubercula quadrigemina not only serve as nervous centres for the perception of light, but a reflex action also takes place through them, by which the quantity of light admitted to the eye is regulated to suit the sensibility of the pupil. In darkness and in twilight, or wherever the light is obscure and feeble, the pupil is enlarged by a relaxation of its circular fibres, so as to admit as large a quantity of light as possible. On first coming into a dark room, accordingly, everything is nearly invisible; but gradually, as the pupil dilates and as more light is admitted, objects begin to show themselves with greater distinctness, and at last we can see tolerably well in a place where we were at first unable to perceive a single object. On the other hand, when the eye is exposed to an unusually brilliant light, the pupil contracts and shuts out so much of it as would be injurious to the retina.

The above is a reflex action, in which the impression received by the retina is transmitted along the optic nerve to the tubercula quadrigemina. From the tubercles, a motor impulse is then sent out through the motor nerves of the eye and the filaments distributed to the iris, and a contraction of the pupil takes place in consequence. The optic nerves act here as sensitive fibres, which convey the impression from the retina to the ganglion; and if they be irritated in any part of their course with the point of a needle, the result is a contraction of the pupil. This influence is not communicated directly from the nerve to the iris, but is first sent inward to the tubercles, to be afterward reflected outward by the motor nerves. So long as the eyeball remains in connection with the brain, mechanical irritation of the optic nerve, as we have

shown above, causes contraction of the pupil; but if the nerve be divided, and the extremity which remains in connection with the eyeball subjected to irritation, no effect upon the pupil is produced.

The anatomical arrangement of the optic nerves, and the connections of the optic tubercles, are modified in a remarkable degree in different animals, to correspond with the position of the two eyes. In fish, for example, the eyes are so placed, on opposite sides of the head, that their axes cannot be brought into parallelism with each other, and the two eyes can never be directed together to the same object. In these animals, the optic nerves cross each other at the base of the brain without any intermixture of their fibres; that from the right optic tubercle passing to the left eye, and that from the left optic tubercle passing to the right eye. (Fig. 143.) The two nervous cords are here totally distinct from each other throughout their entire length; and are only connected, at the point of crossing, by intervening areolar tissue. Impressions made on the right eye must therefore be perceived on the left side of the brain; while those which enter the left eye are conveyed to the right side of the brain.

In birds, also, the axes of the two eyes are so widely divergent that an object cannot be distinctly in focus for both of them at the