Part 3

ARM. │ LEG. I. Shoulder-girdle, consisting │ I. Hip-girdle, consisting of: of: │ Shoulder-blade (Scapula) (Fig.│ Hip-bone (Ilium) (24). 2, 17). │ Collar-bone (Clavicle). │ Pubis. Coracoid process (of Scapula).│ Rump-bone (Ischium). II. Upper arm: │ II. Thigh: Upper arm-bone (Humerus) (18).│ Thigh-bone (Femur) (25). III. Fore arm: │III. Leg: Radius (19). │ Shin-bone (Tibia) (26). Ulna (20). │ Clasp-bone (Fibula) (27). IV. Hand: │ IV. Foot: Two rows of wrist-bones │ Two rows of ankle-bones (Carpal bones) (21). │ (Tarsal bones) (28). Metacarpal bones (22). │ Metatarsal bones (29). Finger-bones (Phalanges) (23).│ Toe-bones (Phalanges) (30).

The differences between arm and leg are explained by their different uses. The bones of the leg, used to support the human body, are firmer and thicker, but less movable than those of the arm, which is employed in grasping. Consequently the union between the hip-girdle and the trunk-skeleton is firmer than that of the shoulder-girdle. The radius can rotate upon the ulna, so as to completely turn the hand over; a similar twisting of the foot would not be of use, and cannot be effected. The leg has a knee-pan (patella) (Fig. 31), with which there is no bone in the arm to correspond. In the foot the toes are short, and the remaining parts long; for instance, one of the tarsal bones, the calcaneum (heel-bone), is strongly developed and projects behind (28*). In the hand, the digits are relatively long, and since the tip of the thumb can be made to touch the tips of all the fingers, are admirably adapted for grasping.

FIG. 2.—The Human Skeleton. ]

The number of fingers or toes is at most five, but may be less. The horse has a single digit to each limb; the ox, two well developed and two remaining as rudiments; the pig, two large and two small; while the dog has four toes in the hind foot, five in the fore foot.

Man walks on the sole of the foot. Some other animals (dog, cat) on the toes; others again (horse, ox, pig), on the tips of the toes. In the last case there is not simply a horny structure (nail or claw) on the upper side of the toe, but a hoof sheathing the whole of its tip. In many animals the thigh and upper arm are drawn close up to the body, so that the limbs appear quite different from those of man. (Compare Fig. 2 with Fig. 3.)

FIG. 3.—Skeleton of an Ox. I. Skull: 1, Frontal bone, with horn cores, a; 2, temporal bone; 3, malar or cheek-bone; 4, maxillary bone; 5, lachrymal bone; 6, nasal bone; 7, premaxillary bone; 8, lower jaw; 9, orbit; 10, occipital bone. II. Neck and Trunk: H, 7 cervical vertebræ; R, 13 thoracic vertebræ; L, 6 lumbar vertebræ; K, sacrum; Su, caudal vertebræ; C, 13 pairs of ribs; D, sternum. III. Fore Limbs: Sc., scapula; A, humerus; S, radius; E, ulna; U, carpus; M, metacarpals; i., ii., iii., phalanges. IV. Hind Limbs: B, hip-girdle, a, ilium, b, ischium; F, femur; P, patella; T, tibia; Sp, tarsus; M, metatarsals; i., ii., iii., phalanges. ]

The bones are usually surrounded by flesh. This consists of a number of different pieces united together by a delicate, elastic, fibrous mass (connective tissue). The different pieces are termed muscles, each of which is again made up of a large number of muscle-fibres, all taking a longitudinal direction. Each fibre can contract, and a muscle becomes shorter and thicker by simultaneous contraction of all its fibres. The contraction and subsequent relaxation of muscles move other parts. There are some muscles, the hollow muscles, which surround a cavity, and by their contraction propel the liquid or solid substances found in their cavity. The heart, for example, is a large muscle of this sort, serving to propel the blood, while the hollow muscular coat of the gut moves on the contained food. Other muscles are fixed by their ends to other parts of the body, which they move by their contraction. We distinguish between dermal muscles and skeletal muscles, attached respectively to the skin or by one end to an integumentary structure (hair, feather, scale), and to parts of the skeleton. The animals which are devoid of any internal skeleton, the invertebrates (i.e. all animals except vertebrates), naturally possess no skeletal muscles. Examples of dermal muscles are those by means of which a bird erects its feathers (tail-coverts of peacock!), and those which enable a hedgehog to roll itself into a ball and stick out its spines. Each end of a skeletal muscle is connected with a bone. If such a muscle contracts the more easily movable bone is drawn towards the less easily movable one (Fig. 4). In order that the bones may be movable upon one another they are united together by joints.

According as muscular movements are, or are not, under the influence of the will, they are distinguished as voluntary and involuntary. To the latter kind belong the movement of the heart, and the movements of the muscles in the wall of the gut by which the food is made to progress.

FIG. 4.—Bending of the Arm by Contraction of the Biceps Muscle. a, humerus; b, ulna; c, elbow-joint; d, biceps muscle; e, origin; f, insertion of the same. In the right-hand figure of the muscle d is contracted; in the left-hand figure it is slackened. ]

To destroy the contractile power of a muscle it is not necessary to injure the muscle itself. Every muscle is related to a nerve, which sends its fine branches to the fibres making up the muscle. If we cut the nerve, the corresponding muscle loses its power of contraction. But the nerve arises from the central nervous system, which in vertebrates principally consists of the brain and spinal cord. The muscle will therefore lose its contractile power if the connection with these central parts is broken. The true cause of movement resides in these parts. A sort of change, the essential nature of which is unknown to us, takes place in them, and is propagated along the nerve to the muscle, causing it to contract. The central nervous system is, therefore, the origin, the centre from which the order to contract proceeds; hence its name. The nerves which run from these central parts to the muscles are known as the nerves of movement (motor nerves).

FIG. 5.—Diagram to explain the Action of the Motor and Sensory Nerves. ]

There is still, however, a second group of nerves, the nerves of sensation (sensory nerves), which arise in the sense-organs (skin, mucous membrane of tongue, nose, ear, eye), and convey to the central nervous system the impressions they receive from the outer world by the aid of these sense-organs. In the appended diagram (Fig. 5), C represents the central nervous system; B.N., a motor nerve, branching in the muscle M; G.N., a sensory nerve, which runs from the blood-bathed inner skin or dermis (L.h.), underlying the outer skin or epidermis (O.h.), to the central system. (The arrows indicate the direction in which impulses are conveyed along the corresponding nerve.)

Men or animals lose in weight if they take no food. The reason for this is that certain substances leave the body either as gases (through the lungs), or as liquids (by the kidneys and sweat-glands), without a corresponding compensation. An animal or human being could not live without taking in fresh substances, which, according as they are solid or liquid, are known as food or drink. The different kinds of food and drink, which, with few exceptions (salts, water), are taken from the animal and plant kingdoms, cannot, however, as such, replace the gradually diminishing body substance, for, to begin with, they contain useless matters, which pass out of the body in the fæces (dung). And even the nutritious parts of the animal and vegetable substances taken into the stomach, are not always in a form in which they can be used at once. Digestion, which in all the higher animals takes place in a food-tube (gut), serves to reduce them to a suitable condition, at the same time separating the useless matters. The action of several fluids (saliva, gastric juice, bile, etc.) secreted by glands, extracts the useful (nutritious) substances from the food and drink, converting them also into a suitable form. The smaller the pieces into which the food is separated, the better can this purpose be effected. In mammals the teeth serve to break down the food; in birds and many Invertebrates the same part is played by special secretions of the stomach or intestine provided with hard ridges.

So long as the nutritious food-stuffs remain in the food-canal, even though in a completely suitable form, they cannot nourish the body. And since waste of the substance of the body everywhere takes place, it is absolutely necessary that the food-stuffs should pass after digestion into a system of organs going to all parts of the body. This system is the circulatory, or vascular system. Food-stuffs enter it from the gut directly or indirectly, reaching it in the latter case through the lymphatic (lacteal) system.

The blood is the fluid into which the food-stuffs are taken up. It consists of an almost colourless liquid, together with an innumerable number of exceedingly minute blood-corpuscles.

The blood flows through the body in a system of tubes, or blood-vessels, which branch repeatedly, and at last become merged in the microscopic capillary blood-vessels. These capillaries are present in nearly all parts of the body except the epidermis and epidermal structures (hairs, feathers, scales, etc.). They have exceedingly thin walls, which present no resistance to the passage of the nutritious substances contained in the blood, so that these can be absorbed by those parts of the body which lie between the individual capillary vessels. The central organ of the circulation is the heart, an enlarged part of the vascular system, possessing thick muscular walls. By contraction of these, the blood is driven out of the heart (Fig. 6, H); and its exit is possible on one side only (a), as at the other side (b) there is a valve, which closes when the heart contracts. The vessel into which the blood leaving the heart enters is termed an artery (S.A.) It divides into several branches, also known as arteries, and the smallest arteries pass into capillaries, which again are connected with veins, which join larger and larger veins, until finally one or a few open into the heart (A).

FIG. 6.—Diagram of the Course of the Circulation. ]

Since the blood in the course of its circulation gives up some of its nutriment to the various parts of the body, it would in the end become useless for the purposes of nutrition if it did not receive a fresh supply of food-stuffs from the gut, either directly or indirectly (through the lacteal system). But apart from this, the blood would ultimately become useless, and that very quickly, if it did not traverse the lungs, kidneys, and sweat-glands. It is well known to every one that a man or animal cannot live without air, or at any rate without a certain gas, oxygen, that is contained in air. This oxygen must be able to penetrate into the minutest particles of the body, and the blood, in the corpuscles of which it is contained, carries it everywhere. In the smallest particles (molecules) of the body an oxidation (combustion) of body substance takes place, which not only causes an evolution of heat, but also renders the body capable of doing work. But if now the blood passes from the capillaries into the veins, it contains too little oxygen. And besides, it has taken up from the molecules of the body several substances, developed in those molecules, which would be fatal to the animal if they were not removed from the body. Now, when the blood streams through the lungs, it gets rid of the poisonous gaseous matter, and when it traverses the kidneys and sweat-glands it parts with the injurious liquid and solid substances. But in the lungs the blood takes up at the same time fresh oxygen; and since in this way the air in the lungs becomes poor in oxygen, the movements of breathing (respiration) provide for the passage of a fresh supply of oxygen into the lungs. Only the higher Vertebrates breathe by means of lungs; fishes and numerous aquatic Invertebrates breathe by gills, and insects by air-tubes (tracheæ).

FIG. 7.—The Small-winged Gall-fly, d (Andricus terminalis), lays its eggs separately in the rootlets of oak. Root-galls (a) result from this, and inside of each of them a larva develops which, after a metamorphosis, becomes a relatively large, wingless gall-fly (c) known as Biorhiza aptera. This pierces the oak-buds in early spring, and lays a large number of eggs in them; from part of the bud is formed a large juicy gall (b), containing several larvæ, from which the small-winged gall flies (d) develop. The species here represented exist, therefore, in two forms, e and d (Heterogeny). ]

While Nutrition is the life-process which shields the individual from death, Reproduction serves to maintain the species. It is familiarly known that the offspring generally resemble their parents. But it is also a fact recognized by the stock-breeder, that a particular animal will not only transmit several of its own characteristics to its offspring, but perhaps also various characteristics of the grandparents or of animals belonging to still more remote generations, although these characteristics are not visible in the animal which is actually breeding (Reversion, Atavism). Among insects and the lower animals there are species which, as adult animals, appear not in one form, but two or several. In this case, as a regular thing, the offspring does not resemble the parents, but the grandparents, great-grandparents, or a still earlier generation. The older observers have placed the offspring and the parents, and sometimes the grandparents too, of the same animal species in different species, or even genera or families, until newer researches on the reproduction and development of these animals have proved them to belong to one and the same species (see Fig. 7 and explanation). The method of reproduction by which a species appears in two or several forms is distinguished as heterogeny and metagenesis, or alternation of generations. In the first (Fig. 7) sexually reproducing animals alternate with other sexual animals. It may be that these are of separate sexes, or else they may possess both male and female organs (hermaphrodite). In metagenesis a sexual generation regularly alternates with one or several generations reproducing asexually.

The animal kingdom falls (cf. p. 2) into sub-kingdoms or main divisions. Seven of these are commonly distinguished: I. Backboned animals; II. Jointed-limbed animals; III. Worms; IV. Molluscs; V. Echinoderms; VI. Cœlenterates; VII. Protozoa.

First Sub-Kingdom: VERTEBRATA (BACKBONED ANIMALS).

The Vertebrate body possesses a bilateral or twosided symmetry; i.e. it can be separated into two exactly corresponding halves, by a plane of division. The bilateral symmetry is strictly carried out as regards the external parts of the body, a single exception to this being flat-fish (plaice, flounder, etc.); but, on the other hand, it is more or less obliterated in the arrangement of the internal organs. In the Vertebrate body we find, as an axis, a vertebral column (backbone) made up of vertebræ, and traversed by the vertebral canal. As soon as this canal widens out in the skull to the cranial cavity, the spinal cord, which it contains, merges into the brain. In addition to the cavity containing the central nervous system, and placed on the upper side (= dorsal side) of the animal, a cavity, the body-cavity, is found in the under side (= ventral side). It contains for the most part the organs of respiration, circulation, digestion, and excretion (Fig. 1), and in Mammals is divided by the diaphragm into thoracic and abdominal cavities. In all the other subdivisions of the animal kingdom the central nervous system is situated in the same cavity as the above-mentioned organs.

Various bones are connected with the vertebral column, and they serve for the attachment of muscles. The bones collectively constitute the skeleton, which is one of the most distinctive features of a Vertebrate.

The animals of this sub-kingdom never have more than four limbs, and their blood is red, while that of most other animal groups is colourless.

The structure of the heart in the various Vertebrates must also be noticed. In no Vertebrate is this organ so simple in structure as in the scheme given in Fig. 6; such an arrangement, moreover, would involve great difficulties. One great difficulty would be that while the blood was leaving the heart at a (Fig. 6), no fresh blood could enter, so that the blood in the veins would stand still. Even in the lowest Vertebrates (the Fishes) this difficulty is obviated, for where the main vein (or veins) opens into the heart an enlargement of this vein is found, where the blood can collect as long as the heart continues to contract. This expansion is also reckoned as part of the heart, and named the auricle (Fig. 8, V.K.), while the heart proper is termed the ventricle (K.). It is also easy to see that there must be a tolerably wide opening between the two chambers, so that as soon as the ventricle becomes flaccid the auricle can force blood into it. But there being such a wide aperture between auricle and ventricle, one valve is not enough to make it impossible for the blood to pass back into the auricle during the contraction of the ventricular walls. There are two or three valves there (Fig. 8, Kl.) fixed by fibres to the wall of the ventricle. In order that the blood which is forced into the artery (S.A.) may not pass into the ventricle during its relaxation, there is another valve (not indicated in Fig. 8), at the base of the artery.

FIG. 8.—Diagram of the Heart in a Fish. ]

An arrangement like that so far described is found in fishes. The heart consists in them of an auricle, into which is returned the blood that has traversed the body, and of a ventricle which moves it on again. But the blood that has traversed the body is on that account poor in oxygen, and consequently unfit to be circulated again when it is returned to the heart. It is necessary for it to take up fresh oxygen before being circulated again. In fishes this difficulty is met by the blood, poor in oxygen, which flows out of the ventricle, first going to the gills and streaming through them. The gills consist of a very large number of small, thin-walled outgrowths arranged in regular rows on the firm gill-arches. The blood, poor in oxygen, passing out of the ventricle and through various arterial branches to the gill-filaments, takes up fresh oxygen as it streams through these from the oxygen dissolved in the water which constantly surrounds them. For this purpose a stream of pure water is regularly taken in by the mouth and expelled again, right and left, through the gill-slits. The blood, having become rich in oxygen in the gills, is now once more fit for circulation through the body, and therefore flows out of the gill-capillaries into larger vessels, which finally unite into a single large vessel that carries the purified blood to the various parts of the body. In the arrangement of the heart here described there is the disadvantageous condition that the blood is obliged to traverse two sets of capillaries (gill and body capillaries). This is not an easy matter, for there is a great deal of friction between the blood and the walls of the capillaries, constituting a hindrance to its progress. The circulation of the blood in fishes is consequently very slow, and since the blood contains the oxygen which is used by the various parts of the body, oxidation goes on slowly in the body of a fish; hence the small amount of heat developed there. Since fishes almost immediately give off to their surroundings the small amount of heat which they develop, they have no constant body temperature, varying in this respect with the temperature of the surrounding water. Such animals are termed cold-blooded.