Cerebellum.—The functions of this organ have been made the subject of much dis cussion and investigation. It is itself insensible to irritation, and has been cut away in various animals (by Longet and other French physiologists), without eliciting signs of pain; moreover its removal or disorganization by disease is generally unaccompanied with loss or disorder of sensibility, animals from whom it has been removed being appar ently able to smell, see, hear, and feel, as perfectly as before. Flourens seems by his vivisections to have arrived at the correct view regarding the functions of this organ, and his results have been fully confirmed by Longet and others. He extirpated the C. in birds by successive layers. Feebleness and want of harmony of the movements resulted from the removal of the superficial layers; when he reached the middle layers, the animals became restless; their movements were violent and irregular; but they were not convulsed, and their sight and hearing were perfect. By the time that the organ was entirely removed, the animals had completely lost the power of flying, walking, standing, and preserving their equilibrium. When a pigeon in this state was laid upon its back, it could not recover its former position; but fluttered its wings, and saw and tried to avoid a threatened blow. Hence volition, sensation, and memory were not lost, but merely the faculty of combining the actions of the muscles. From a large series of experiments of this kind, subsequently made on all classes of animals, Flourens infers that the C. belongs neither to the sensitive nor to the intellectual apparatus ; and that it is not the source of voluntary movements, although it belongs to the motor apparatus; but that it is the organ for the co-ordination of the voluntary movements, or for the excitement of the combined and harmonious action of the muscles.
This view is confirmed by the phenomena observed in certain cases of disease, and to a certain extent by comparative anatomy, for to each of the four classes of vertebrata if we reckon amphibians and reptiles as a single class—the species whose natural move ments require the most rapid and exact Combinations of muscular actions are those in which the C. is most developed in proportion to the spinal cord; and if we compare different species of the same class, we usually find the development of the C. to corre
spond very closely with the perfection and variety of the muscular movements. For example, in the frog the movements are exceedingly simple in character, consisting of little else than flexion and extension of the posterior limbs; and the C. of this animal is extremely small compared with the rest of the brain, being merely a thin narrow band of nervous matter. In the common sea-turtles, the movements of the body arc of a more varied character, and the motions of the head and neck are more extensive; and here we have a much more highly developed cerebullum. In the alligator, again, a reptile whose motions closely resemble those of quadrepeds, the C. is still more fully developed.
The influence of each half of the C. is directed to the muscles of the opposite side of the body, and for the right ordering of the movements, the actions of its two halves must be mutually balanced and adjusted; for if the nervous structures uniting one of the halves of the C. with the medulla oblongata and spinal cord be divided, strangely disordered movements bCcur, the Animal ,dowu on,the side opposite to that which has been injured, and continually rotating round the long axis of its body, sometimes for several days, at the rate of fifty or sixty times in a minute. Similar movements have been observed in men in whom one of the crura of the C. has been diseased.
Phrenologists are of opinion, in accordance with the view originally propounded by Gall, that the C. is the seat of the sexual impulse and instincts; but this view has been long abandoned by almost all physiologists, for the reason that it has not been found to be sufficiently supported by anatomical and experimental facts, many of which are indeed directly opposed to it.
Our limited space compels us to leave altogether untouched many most interesting topics in cerebral physiology, as, for instance, the quality of the brain, the plurality of the cerebral organs, etc. The reader who wishes for further information, is referred to Kirkes's Physiology (from which we have freely quoted), Carpenter's Human, Physiology, Noble On the Brain, Holland's Chapters on Mental Physiology, and Brodie's Psychological inquiries.