The cranium, formed of many bones firmly united together, consists of a cerebral region, or box, containing and guarding the brain, and a facial region, in which are situated, besides the mouth, the organs of sight and smell. The bones connected with the mouth are the two maxillæ, along the margins of which are placed the grinding- or cheek-teeth; the two pre-maxillæ, in which are set the cutting- and the eye-teeth; and lastly, the palatine bones which form the roof of the mouth. Hinged on to the sides of the cranium is the toothed mandible, or lower jaw, composed of two halves, which may be solidly or loosely joined together in the mid-line, or symphysis. Along the under surface of the skull, there are, besides the great (often posterior) orifice for the entrance of the spinal cord, numerous foramina, or openings, for the passage of blood-vessels for the nourishment of the brain, and of nerves which bring all parts of the body into relation with the supreme directing centre. Conspicuous near its posterior part, on each side, is an ivory-like capsule, the periotic bone, containing the essential organ of hearing. Lying beneath the lower jaw is the hyoid arch, a slender framework of bones, supporting the tongue and the upper end of the windpipe with the organ of voice. In a few of the Monkeys and Apes certain of the bones of this arch are much enlarged and hollowed for increasing the volume of sound emitted by them. On either side of the great opening which is so conspicuous at the hinder part of the skull, for the reception of the spinal cord, is a smooth kidney-shaped surface, called a "condyle." These two condyles serve for the articulation of the first segment of the back-bone to the cranium, and by the possession of this pair of condyles the Mammalian skull can always be distinguished from that of Birds and Reptiles. The pieces of which the back-bone are composed are named the vertebræ. Those of the neck, the "cervical" vertebræ, are recognised by having no true ribs attached to them, and are, in all Primates, seven in number. Those of the back, or "dorsal" vertebræ, may be distinguished by having articulated to them, on each side, a movable rib, the other end of which is attached to the breast-bone; they follow next to the cervical vertebræ, while to them succeed the "lumbar" vertebræ which carry no complete ribs. The dorsal and lumbar segments vary in number, but together they rarely exceed seventeen. Behind these extend the "sacral" vertebræ—completely ossified together, and lastly, the bones of the tail or "caudal" vertebræ, which may be many or few, according to the length of that appendage.
The fore-limb is composed of three segments, the arm, fore-arm, and hand, together with a block by which it is attached to the side of the body. To this block—the blade-bone or scapula—is articulated the arm-bone, or humerus, which at its elbow-joint hinges with the two bones, the ulna and the radius, of the fore-arm, on which in turn the hand is rotated. The hand is made up of three parts, the wrist-bones, or carpus, closely united together in two transverse rows with a central bone intervening between them; next the elongated bones of the palm of the hand, or metacarpus, one to each finger, and lastly the phalanges, or finger-bones, three to each digit, except in the thumb, where there are but two. The hind-limb is formed on exactly the same plan. It has a connecting block—the pelvis; giving suspension to the thigh, with its single bone, the femur, to which articulates the leg, with two bones (tibia and fibula), and the tripartite foot, composed of tarsus, metatarsus, and phalanges.
Of the digestive organs of the Primates the teeth present very important characters, from the point of view of the classification of the Order. They differ in form and number, and have distinct functions to perform. The teeth situated in front are the incisors and canines, sharp and pointed, for seizing, cutting, and holding the food. Behind them come the pre-molars, and still further back the molars, both with broad crowns of complicated tubercles and ridges for milling the hard portions contained in the food. Animals provided—as all the Primates are—with these different sorts of teeth, are said to be Heterodont,[[2]] in contradistinction to forms like the Dolphins and Whales, which are termed Homodont,[[3]] because the whole of these teeth are of the same pattern. The Primates are Diphyodont[[4]] as well, because many of their permanent teeth are preceded by another set, commonly known as the milk-teeth. In order to present to the eye at a glance the number of each sort that any species possesses, a dental formula has been adopted by naturalists. Such a formula as I22, C11, P33, M33 = 36, indicates that in one half of the mouth, above and below, there are 2 incisors, 1 canine, 3 pre-molars, and 3 molars = 18; and therefore in the two halves of the mouth together there are 36 teeth in all.
The masticated food, partially digested by the saliva of the mouth, descends the gullet by the muscular contractions of its walls to the simple, sac-like, stomach, and thence to the intestines. These latter consist of two portions, one smaller and narrower, nearer to the stomach, and a second portion further down, larger and wider; the junction of the two portions being marked by a process of varying length, the cæcum. The stomach and intestines, with other important structures, such as the liver, kidneys and generative organs, are contained in a lower cavity, separated by a muscular midriff, the diaphragm, from the upper part or thorax, containing the blood-purifying and pumping organs, the lungs and the heart.
The upper part of the windpipe is, in all Primates, modified to form the larynx, or organ of voice, constituted by fibrous strings stretched across its orifice, where they may be set in vibration by the air, in its passage to and from the lungs.
The brain is relatively large in proportion to the body, and attains in the higher of the two sub-orders its most perfect development. The main brain (or cerebral hemispheres), when viewed from above, in size preponderates over, and conceals (except in the Lemurs) all the other parts of that organ. The surface of its lateral halves, which are connected by transverse bands so as to insure harmony of action between them, is marked by fissures and foldings, or convolutions, which vary in number and complexity, evidently in relation to the intelligence of the animal. The brain within the skull gives origin to the nerves for the chief organs of sense; while from its posterior part it is continued along the back—within a canal formed by the neural arches of the vertebræ—as the spinal column, from which arise the rest of the nerves for the body.
The young of all the Primates are nourished in the mother's womb by the passage of material from the blood-vessels of the parent through an organ known as the placenta. They are all born in a helpless condition, and remain unable to look after themselves for a considerable period, during which they are dependent on the milk secreted on the ventral surface of the mother by two or four glands, the teats or mammæ—those characteristic organs from which the "Mammalia" have derived their name. These glands are present in both sexes, but are functional only in the female.
We shall now proceed to describe more minutely the first of the two sub-orders of the Primates—the Lemur-like animals.
I. THE LEMURS—SUB-ORDER LEMUROIDEA.
The Aye-Aye, the Tarsier, and the True Lemurs constitute this first sub-order. They are characterised by having the muzzle long and narrow, more or less Dog-like in shape, and the upper lip often divided into two by the nose-pad. The external ears (Fig. 1) are enlarged, with flattened margins, but have no "hem" as in the higher Anthropoids. (Fig. 2.)