Primates

PRIMATES, an order of mammals, including man, the apes, monkeys, tarsiers and lemurs. The name, meaning "chiefs," was given by Linnaeus in 1758. Linnaeus also included the bats, assigned to a separate order by modern zoologists.

Structure and Habits.

Primates are primarily arboreal ani mals with hands and feet adapted for climbing. The fingers and toes are provided with nails (rarely compressed as claws). The great toe of the hind-foot is usually more or less divergent and the foot functions as a grasping organ, branches of trees being seized between the great toe and the four outer toes. The hands serve both for climbing and for manipulating food. The limbs are relatively long and slender, with free movements of rotation and supination. The lower primates (lemurs) are prac tically quadrupeds that run and leap in the trees but in the more advanced monkeys and apes there is an increasing tendency to climb with the arms extended above the head and the weight of the body suspended beneath the branches (brachiation). On the other hand, in the baboons the limbs plainly show secondary adaptation for running on the ground, both in their somewhat dog-like proportions and in the reduction of the great toe.

In the stem type of arboreal monkeys, vision is dominant over smell, whereas in typical ground-dwelling mammals the reverse is the case. Hence the occipital poles of the brain, which are con nected with vision, are much enlarged. The adjustments for bal ancing in such actively climbing animals are extremely various and rapid ; this requires a correspondingly complex development of the cerebellum and of the cerebral areas of the brain concerned with the movements of the limbs and body. As intelligence in creases the prefrontal lobes of the brain develop. Thus the brain of monkeys is proportionately larger and more complexly con voluted than in ordinary ground-living animals. As a whole the brain of the Old World monkeys presents the ground-plan of the human brain without the special developments and complications connected with man's superior mentality.

The bony braincase closely follows the shape of the brain, ex cept in front, where the large eye sockets jut forward, thus en abling the achievement of binocular, stereoscopic vision. Owing to the forward growth of the temporal lobes of the brain the greater wings of the sphenoid are moved forward to form the back-wall of the orbits, which are thus separated by a bony partition from the temporal fossae.

Hearing is acute and the temporal lobes are large. A thin bony shell, the auditory bulla, on the under-side of the braincase behind the socket for the lower jaw, covers the lower side of the cavity of the middle ear and is connected by a bony tube with the root of the external ear; the latter resembles that of man but usually has a point on its upper rim and lacks a lobule below.

In the more primitive primates, e.g. lemurs, the jaws are long and slender and the muzzle pointed. In typical monkeys, however, the jaw is shortened and deepened and the muzzle broad, the nose and lips assuming more or less the human aspect. The opposite halves of the lower jaw are fused in front even in young animals. The dentition is adapted for a mixed diet, with fruits or vegetation prevailing. The teeth in an adult Old World monkey number 32, as in man. The incisors are cutting teeth, the lower ones slightly inclined forward. There are two pairs in both upper and lower jaw. The canines are sharp and adapted for biting; the upper premolars are bicuspid, i.e., with single, outer and inner cusps; and the low-crowned molars are surmounted by low cusps.

The female reproductive organs are fundamentally as in man (except in details) and there is likewise only a single pair of breasts in the female. The placenta is disc-shaped and intimately attached to the wall of the uterus.

Thus a typical monkey differs from an ordinary mammal such as a dog in its thorough adaptation to arboreal life, in the greater activity of the visual as compared with the olfactory powers, in its more advanced type of brain, and in its much greater likeness to man in the entire ground-plan of its anatomy.

Such a monkey is structurally connected on the one hand with lower primates (including New World monkeys, tarsiers and lemurs) and on the other hand with the higher primates, the anthropoid apes and man. Even the still existing species of pri mates form a fairly gradual transition from the tree-shrews at the base to highly specialized forms like the spider-monkey, the orang-utan and man, which stand far out on widely divergent branches.