Even in this Monkey, which is low in the scale on account of its having badly-developed thumbs, the structures of the brain greatly resemble those of the Monkeys of the Old World. The nerves are large in proportion to the substance of the brain, and the brain proper is narrow in front and hollowed out beneath, where it rests on the orbits. But these proofs of a low kind of intelligence and of great muscular power are accompanied by structures which mimic or sketch out those of the human brain in an extraordinary manner. The cerebellum, or little brain, is large, as it is the organ which has much to do with regulating and co-ordinating the movements of the muscles, but it is quite covered by the back part, or posterior lobes, of the brain. Inside the brain the cavities called the ventricles may be seen, and they are made on the human plan, for the cavity on each side (lateral ventricle) has a front part, a back part, and a deep one, and on its lower surface, or floor, certain roundings, which are called by odd names, such as the hippocampus minor and the hippocampus major. These are visible in the brain of Ateles as they are in man. Now, it is very remarkable, allowing for the difference in the size of the brain of most other Monkeys and of man, that the Spider Monkey should have larger posterior lobes to its brain than they have. Moreover, this unusual size produces a greater length of the back part (or horn, or cornu) of the lateral ventricle in Ateles. The difference, however, between the packing of the nervous substance of the brain in man and in the Spider Monkey is vast, for in this last there are few convolutions, but the principal are happily said by Huxley to sketch out the position of the most important in the human brain. The projection of the back part of the brain of the Spider Monkeys over the cerebellum is at least one-tenth of an inch. Hence there is much nervous matter in the back part of the brain, and this compensates for the narrowing and diminution of nervous matter in the front. Are the nerves, then, which give the Spider Monkey its wonderful power of activity and complicated movement, situated in the back part of the head? At present physiologists have not satisfactorily shown what are the offices of these back or occipital lobes of the brain; the rounded floor of the cavity in the brain, which goes by the absurd name of hippocampus, because it is curved like a “sea horse,” and which is well seen in Ateles, has much to do with the sensation of touch, and the nervous matter at the sides of the brain appears to be connected with the nerves of the muscles of the limbs. The Ateles lead a life of very great sameness in their forests, and their perceptions and intelligence are never greatly stimulated, hence the fore part of the brain is small. [Illustration: HAND OF THE SPIDER MONKEY.] THE COAITA.[84]