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SIMPLIFICATION OF STRUCTURE IN LOWEST ANIMALS.

substance upon the supply of blood which it receives, is shown by the fact, that if this supply be temporarily cut off, either by failure of the heart's action (as in fainting), or by pressure on the blood-vessels which convey it, immediate insensibility, with loss of all power of motion, is the result. And the same is the case with regard to the organs of sense ; for if the circulation through them be interrupted, no sensory impression can be made upon the nerve-fibres which originate in them, as we see when the movement of blood in a limb is suspended by pressure upon its artery.

64. The foregoing constitute the principal tissues among the higher animals, in which the principle of division of labour is most fully carried out, every component part having its own peculiar structure and its own special action. As we descend in the scale, we find these distinctions less and less obvious, so that when we come down to Zoophytes (§ 121), we meet with but little differentiation either in the textures or in the actions of the several parts of the body; the whole substance of these animals being composed of a tissue, which very closely resembles that which is first formed in higher animals for the reparation of wounds, having the appearance of a solidified blastema (§ 34), with nuclear particles, in various phases of development into cells and fibres, more or less thickly scattered through it; and this substance being everywhere contractile, and everywhere (at least in many instances) equally capable of participating in the functions of nutrition and reproduction. And when we pass still lower, to that simplest type of animal life, which is presented to us in the Rhizopods (§ 129), we do not meet with even this amount of definite structure, but find the entire substance of their bodies composed of an apparently homogeneous jelly, which, like the more organized tissue of the Zoophytes, is everywhere contractile, and which has also the power of performing every operation required for its growth and maintenance as a living being. In such creatures there is not the slightest vestige of a Nervous system; and it remains a question whether, in consequence of this deficiency, they are altogether destitute of consciousness, or whether this endowment is diffused, as it were, through the whole substance of their bodies.

65. Every component part of the fabric must be regarded

INDEPENDENT VITALITY OF PARTS OF ORGANISM.

8i

as having a life of its own, which it maintains by drawing to itself the nutrient material supplied by the circulating current; but as the continuance of its vital activity is dependent upon the continuance of its nutrition, the life of no tissue can be prolonged for any considerable period after the circulation has ceased. But after the movement of the blood has come to an end, though the body as a whole is dead, its parts may remain alive for a certain time, and may perform their functions, so long as they are supplied with the necessary materials. Thus, various secretions, the growth of hair, and muscular movements, have been observed to take place in dead bodies. But they cannot continue, because the necessary conditions are withheld by the stoppage of the circulation,-a function which thus binds, as it were, into one whole the scattered elements, and causes the different operations to minister one to another. As every component part has an independent life, so has it a limited duration, quite irrespective of that of the organism as a whole. Thus the cells which float separately in the blood, seem to be continually undergoing change,-dying, and giving place to new ones. We have seen that the cells of the epidermis and of some parts of the epithelium are being constantly thrown off and renewed. The duration of the cells of fat and cartilage appears to be much greater; in fact, we have no precise knowledge of their term of life. That of the bony tissue is probably greater still; yet there is adequate evidence that it is by no means indeterminate. But that of the muscular and nervous tissues seems to depend almost entirely on the use that is made of them. Thus we may justly say,-however startling the assertion may seem,-that death and decay are continually going on in every living animal body, and are essential to the activity of its functions.

66. Many animals are reduced to a state of apparent death by dryness, by cold, or by exclusion of the air. A curious example of the first kind is furnished by the Tardigrada (ZOOLOGY, § 841); some species of which may not only be completely dried up, but may even be exposed in that state to a temperature much exceeding that of boiling water, without losing the power of recovery when moistened. A similar power of revival after being dried up is possessed by the common Wheel Animalcule, and probably also by the

G

82

SUSPENDED ANIMATION.

eggs of many minute Entomostracous Crustacea (ZOOLOGY, §§ 883, 931). It is unquestionable that many Fishes, especially those of fresh-water lakes, will revive on being thawed after having been completely frozen; and the same has been ascertained of certain Caterpillars. The Snail, when retiring for the winter, seals the orifice of its shell with an impervious lid; and in this cavity it may remain shut up for years, until re-excited to activity by warmth and moisture. Animals in such states of torpidity strongly resemble seeds that are prevented from germinating, apparently for unlimited periods, by being kept at a moderate temperature, and excluded from the influence of air and moisture, which, with adequate warmth, would call them into active growth, but which, at a lower temperature, would occasion their decomposition. There are no positive facts which enable us to say how long Animals may remain in a parallel condition; but there seems no reason why it might not be indefinitely prolonged.

67. The death of the body, then, does not consist in the mere suspension of its vital activity; for so long as that activity may be renewed when the requisite conditions are supplied, so long must the organism be considered as alive, however death-like its condition may seem. Among warmblooded animals, such a suspension, if complete, cannot be endured for more than a very brief period, without the extinction of life; for the substance of their tissues is so prone to decomposition, that it speedily passes into decay unless prevented from doing so either by a reduction of temperature, or by complete drying-up, or by entire seclusion from air; and although each of these methods, practised upon animal substances already dead, may prevent the occurrence of decomposition for almost unlimited periods, yet neither can be applied to the living tissues of any of the higher animals, without occasioning the entire loss of their vitality, as we see (in regard to cold) in the loss of members by "frost-bite." Such parts die, because not only is their vital activity suspended, but their vital properties are annihilated. Their death, however, does not necessarily involve that of the organism as a whole; since the stoppage of their function may not disarrange the general train of vital operations, or their duty can be discharged by other organs. And among many of the lower animals, we find that there is a provision

DECAY CONSTANT DURING LIFE.

83 for their replacement by ordinary acts of growth; and that even when the body has been so severely injured that the organic functions are seriously disturbed for a time (as when a Hydra is divided into two or more pieces, § 122), the vitality of the individual parts is sufficiently enduring, and their reparative powers sufficiently energetic, to enable them to reproduce all that is wanting for the completion of the organism, and for the renewal of its ordinary actions. Among the higher animals, the death of the organism at large may be said to take place when the circulation finally ceases; since, as we have just seen, every individual part must ere long lose its peculiar functional activity, and the entire body be subject to decay.

68. From what has been stated, it will be seen that Life cannot be regarded as a condition in which decay is resisted; for an incessant decay is taking place in every living organism as a necessary condition of its vital activity, being only checked when that activity is itself suspended. But it is a condition in which, by the wonderful harmony and mutual adaptation of the operations of the different parts, the reparative action of the Organic Functions is made to countervail the destructive action involved in the exercise of the Animal Faculties; whilst the latter, in their turn, serve to furnish the conditions requisite for the maintenance of the former. So long as all these actions go on with regularity and completeness, so long the whole body lives; but if any one of the more important among them be interrupted, the stoppage of the whole is the result. This relation of mutual dependence is most intimate in the higher animals; in which, by the differentiation of the several tissues and organs, and the specialization of their functions, the division of labour is carried to its greatest extent, so that no part can entirely fulfil the duty of any other. On the other hand, it is among those lowest forms of animal life, in which there is the greatest multiplication of similar parts, and the greatest diffusion of the same endowments amongst them all, that we find the dependence of the several parts of the organism upon each other to be the slightest, and severe injuries to be tolerated with the least general disturbance.

84

PRINCIPAL TYPES OF ANIMAL STRUCTURE.

is

CHAPTER II.

GENERAL VIEW OF THE ANIMAL KINGDOM.

69. WHEN We examine the Animal Kingdom as a whole, it easy to distinguish in it four general plans or types of structure, by which, with almost infinite variations in detail, the formation of the several beings that compose it has been guided. As specimens of these four plans or types, we may name four animals which are familiar to almost every one,the Dog, the Lobster, the Snail, and the Star-fish. The dif ferences by which these types are distinguished, are manifested in the arrangement of the different organs of the body; and particularly in the form of the nervous system and its instruments. It has been already stated (§ 4) that the power of feeling, and of spontaneous motion, is that which peculiarly distinguishes the Animal from the Plant; and as these powers are possessed in very different degrees, and exercised in very different modes, by the various tribes of animals,—whilst the operations of nutrition are performed, as in plants, in a much more uniform manner, they afford us a satisfactory means of separating these tribes from one another. For the nervous

system is the organ to which these powers are due; and we find it presenting forms so different in the four great divisions already alluded to, that we can at once distinguish them by this alone, even where (as sometimes happens) there may be such a blending, in a particular animal, of the general characters of two of them, as to lead us to hesitate in assigning its precise place in the animal kingdom.

70. The highest of these four divisions is that denominated VERTEBRATA, or Vertebrated Animals; it receives its name from the structure characteristic of it,-the possession of a jointed back-bone or vertebral column,-which will be presently described. This is the group to which Man belongs; and all the animals it contains bear a greater or less resem blance to him in structure. We notice in regard to their external form, that they are alike on the two sides of their body; every part having its fellow on the other side. This