Part 2

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 {4}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, {5}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 {6}_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 I2/2, C1/1, P3/3, M3/3 = 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.

{7}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.

{8}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.)

The trunk is relatively long and compressed, and the tail when long is never truly prehensile. Of the limbs, the posterior are longer than the anterior, and all have five digits, each bearing a flat nail except the second toe, which has invariably a long pointed claw, their tips ending in prominent discoidal tactile pads. (Fig. 3.)

Of the digits, the index is sometimes quite rudimentary, while the thumb is large, and the great toe especially so, both being opposable. Teats occur on the breast, on the abdomen, or on both.

Of the skeleton, the eye-sockets, or orbits, are directed forward, and have complete bony margins, which, however, are not {9}closed in by bone behind (as in Monkeys), but freely communicating beneath the post-orbital process (except in _Tarsius_) with the temporal hollow behind. In the young of some species the orbit is more enclosed than it is in the adult: the orifice for the lachrymal duct of the eye is placed external to the margin of the orbit: the hollow for the olfactory lobes of the brain is always large.

Having four kinds of teeth, and a set in succession to the milk-teeth, they are Heterodont and Diphyodont. The dental formula is I2/2, C1/1, P3/3, M3/3 = 36 (_vide_ anteà, p. 6), and the upper jaw has a toothless space in the centre (except in the Aye-Aye). Of the upper teeth, the _incisors_ are sometimes absent, but generally present; if unequal in size the inner one is the larger of the two. The canines are prominent; the pre-molars all have a _cingulum_, or girdle, round the base, more or less enlarged backwards into a process ("talon" or "heel"); the anterior pre-molar vertically long and canine-shaped; the median and posterior with three main points (tubercles or cusps) and one or two smaller ones on the crown, and having a bar or ridge uniting the front inner with the hind outer cusp. The anterior and median _molars_ have three or four main cusps, and one {10}or two smaller or subsidiary ones on the crown; the cingulum is well developed. The posterior molars have generally three cusps.

In the lower jaw the _incisors_ are close-set and comb-like, remarkable for protruding in front, like the teeth of a Rat or a Rabbit. The _canines_ also protrude horizontally, and, being placed alongside of the incisors, are difficult to distinguish from the latter excepting that they are broader and thicker.

Of the _pre-molars_ the anterior are canine-shaped, the median and posterior ones have three main, and one or two subordinate, cusps on the crowns. In both the upper and lower _molars_, cross-bridges stretch between the outer and inner front cusps as well as between the outer and inner hind cusps. {11}There is an oblique ridge between the hind outer and the front inner cusp, and another is often present between the front outer cusp and the anterior "heel," producing, as Huxley has pointed out, almost a double crescentic pattern, as in many lower Mammals. The posterior molar has four or five cusps.

Of the _milk-teeth_, the incisors in the upper jaw change first. Of the molars, two are developed before the change of the pre-molars. In the lower jaw the incisors change first, and when two or three pre-molars have developed the last molar has still to come.

The arm-bone, or _humerus_, has one perforation (_entepicondylar foramen_) on its inner margin, and another above the joint (except in _Perodicticus_). The bones of the fore-arm (_radius_ and _ulna_), and those of the leg (_tibia_ and _fibula_) are not co-ossified (except in _Tarsius_), so that the palm or sole can be turned up at will.

The bones of the _digits_ are more or less flat and rounded at the tips (differing in this respect from the _Insectivora_). One of the ankle-bones, for the articulation of the opposable great toe, the ento-cuneiform, as it is called, is rounded, as in the Anthropoid Apes and Man. The thumb is opposable, but its articulating bone in the wrist is not rounded, except in _Avahis_ and _Indris_, which genera agree in this respect with _Anthropopithecus_ and Man. The wrist has its central bone (_os centrale_) present; it is absent in Man and the higher Apes.

The knee is free and not united to the side of the body by integument.

The two halves of the lower jaw are not always co-ossified (as is the case in the _Anthropoidea_).

The opening in the base of the skull (the _foramen rotundum_) which transmits from the brain a branch of the fifth nerve {12}for the upper jaw, and the sphenoidal fissure, which gives exit to the third, fourth and sixth cranial nerves, have but one aperture, as in the Rabbit, which belongs to the _Rodentia_.

The sacral vertebræ are generally three in number, and the lumbar and dorsal together vary from nineteen to twenty-three.

The brain, as Sir William Flower has observed, departs considerably from the form of what may be called the primatial type, and approaches in form to that of the carnivorous animals. The hind-brain, or _cerebellum_, is not completely covered by the cerebrum. The latter has but few convolutions (indicating a low intelligence), but its posterior lobe is always present, though more or less rudimentary, and so also are many fissures, which are characteristic of its surface in the higher Primates. The olfactory lobes are usually large and not covered by the cerebrum.

The uterus and structures for the nutrition of the young prior to birth are low in type, and approximate to the conditions seen in the Pig, the Horse, the Chevrotains, and the Ruminants. The unborn Lemur is often encased (as among the Sloths) in a skin-like covering (_epitrichium_) which breaks into patches before birth.

The tongue has a horny supplementary under-tongue (_sublingua_) attached beneath it. The stomach is simple, not formed of several compartments. The transverse portion of the great intestine is convoluted in a remarkable manner upon itself, the cæcum also being very large. The main arteries of the arm and leg break up (as in the Sloths) into an immense number of small vessels (called _retia mirabilia_) parallel to one another instead of being simple branching trunks.

The long tendons of the muscles for flexing the digits (the {13}_flexor longus digitorum_) differ generally in arrangement from those of the higher Primates.

The Lemuroids are of no commercial value to Man.

As regards their distribution, the _Lemuroidea_ are now absolutely confined to the Old World, and predominate in the island of Madagascar, where, as M. Grandidier remarks in his magnificent work on that country, there is scarcely a little wood in any district in which they are not found. Indeed, of the nearly seventy species of Mammals inhabiting that island, thirty-five, or one-half, are Lemurs. Members of the family also occur across the whole of the neighbouring continent of Africa, but their northern range does not reach quite to the tropic, whereas it extends some few degrees beyond it in the Southern Hemisphere. Elsewhere they are confined to the forests of the Oriental region. More or less isolated in Southern India, they re-appear in China, and spreading south to Java they reach as far east as Celebes and the Philippine Islands. The present isolation of the Lemurs in two such distant areas--in Africa and Madagascar and some of the Mascarene Islands on the one hand, and in Southern India, China, Ceylon, and the Malayan Islands on the other--has been considered by some naturalists as weighty evidence in favour of a former land connection between these distant regions.

Though so restricted in their distribution at the present day, this group was more widely represented in past ages of the world's history, as we shall have to point out later on. Abundant fossil remains prove that they lived in Europe and in North America, where to-day they are quite unknown.

The _Lemuroidea_ are almost entirely arboreal, and seldom come to the ground, except the Sifakas, which then progress {14}on their hind legs by a series of bounds, holding their hands over their head in a ludicrous fashion. Most of them are nocturnal, or crepuscular, sleeping the greater part of the day in holes or on a branch of a tree coiled up in a ball. Their food consists chiefly of leaves, fruits, honey, birds' eggs, and birds, or any small animals they can pounce upon.

The Lemurs now living are divided into three families. The Aye-Aye and the Tarsiers, on account of their very special characters, constitute each a distinct family--named _Chiromyidæ_ and _Tarsiidæ_ respectively--while the True Lemurs form the third, the _Lemuridæ_, to which all the remaining forms belong.

THE AYE-AYES. FAMILY CHIROMYIDÆ.

This very aberrant family contains only one species; the characters of the family and of the genus _Chiromys_ are, therefore, necessarily those of the single species known.

THE AYE-AYE. CHIROMYS MADAGASCARIENSIS.

_Sciurus madagascariensis_, Gmel., S. N., i., p. 152 (1788).

_Daubentonia madagascariensis_, Geoffr., Décad. Philos., iv., p. 193 (1795); Dahlbom, Studia, p. 326, t. 12.

_Chiromys madagascariensis_, Cuv., Leçons d'Anat. Comp., Tabl. de Class., 1 (1800); Owen, Tr. Z. S., vol. v., p. 33; Peters, Abhandl. K. Akad. Berlin, 1865, p. 79.

(_Plate I._)

PLATE I.

{15}CHARACTERS.--Head short and round; face short-snouted, with a patch of bristles below the eye, between the ear and the angle of the mouth; eyes round, prominent; eyebrows long and bristly; pupils wide, furnished with a false eyelid (a nictitating membrane); ears large, rounded, directed backwards, naked, and studded with small protuberances; tail longer than the body, bushy, with hair 3-4 inches long; hind-limbs longer than the fore-limbs, the thigh-bone being one third longer than the humerus, the hand the longest segment of the fore-limb; fingers long--the fourth the longest--with compressed and pointed claws, which are proportionately much longer than the toes; the middle or third digit slender and very remarkable, being extremely attenuated and wire-like; thumb opposable, and placed at an acute angle to the short index; great toe opposable, set at an open angle to the other digits, its nail flat; the remaining toes with pointed compressed claws (like the second toe of _Lemuridæ_ and second and third of _Tarsiidæ_). Teats, two, placed low down on the abdomen. Length of body and tail together 36 inches. Skull highly arched, convex transversely; muzzle short and deep; bony palate not extending behind the middle of the posterior molar tooth; lower jaw with condyle elongated from before backwards and on a level with the cheek-teeth, its two halves united at an acute angle by elastic tissue, allowing each half to play independently of the other. Its dental formula, I1/1, C0/0, P1/0, M3/3 = 18. Incisors very large, curved, with persistent pulp, and enamel only in front, growing up as fast as worn away; canines absent (the last two characters as in the Rodents); long vacuity between incisors and pre-molar; pre-molar much smaller than molars; molars with flat crowns and very indistinct tubercules; milk-teeth agreeing more in number and form with those seen among Lemurs than with the permanent set; the upper jaw having its full set of two incisors, one canine, and a pre-molar tooth present; the lower jaw having one incisor, no canine, and one pre-molar tooth on each side. Dorsal and lumbar vertebræ together 18, sacral 3, and caudal 22-27.

{16}Olfactory lobes of brain covered by the cerebrum; convolutions and grooves of cerebrum similar to those in normal Lemurs. Intestine 26 inches long; no striped tissue in the muscular sheath of the gullet at the anterior end of the stomach. Digastric muscle (for moving the jaws) very much developed in accordance with the great gnawing powers of the species.

Fur on back, flanks, tail, and limbs dark brown, nearly black, but with the white of the basal half of the hairs shining through; hair woolly at base; long hairs on top of head and back of neck tipped with white; short hairs of face dirty white. Nose and lips naked, flesh-coloured; ears black; sides of head and throat greyish-yellow; chest often bright yellow, the chin paler. Inner sides of limbs yellowish-white, and on the under surface of the body the basal part of hairs showing through, producing a pale yellowish-white, or sub-rufous, colour. Feet and digits black. Tail black, at its base greyish-white or greyish-brown, and often with long white hairs throughout. The species is more nearly related to the members of the genus _Galago_ to be described later on, than to any other of the _Lemuroidea_.

DISTRIBUTION.--The Aye-Aye is confined to the island of Madagascar. It makes its home in the dense parts of the great forest that runs along the eastern border of its central plateau, but only in that part of it which separates the Sihànaka Province from that of the Betsimisàraka, which is about 25 miles from the east coast, in latitude 17° 22[prime] S. It is more common than has been supposed, its noctural habits and the superstitious awe with which it is regarded accounting for its apparent rarity, and for the contradictory reports given of its habits.