5. In the last place, structure is complex in proportion to the energy of function. The greater the power with which voluntary motion is capable of being exerted, the higher the organization of the apparatus by which it is performed; the more compact and dense the shell, the cartilage, the bone, the firmer the fibre of the muscle, and, in general, the greater its comparative bulk. The wing of the eagle is as much more developed than the wing of the wren, as its flight is higher, and its speed swifter. The muscles which give to the tiger the rapidity and strength of its spring possess a more intense organization than those which slowly move on the tardigrade sloth. The structure of the brain of man is more exquisite than that of the fish, as his perceptions are more acute, and capable of greater combination, comprehension, and continuity.
Thus we see that the organization of the animal is more complex than that of the plant, not from an arbitrary disposition, but from absolute necessity. The few and simple functions performed by the plant require only the few and simple organs with which it is provided: the numerous and complicated functions performed by the animal require its numerous and complicated organs: the plant, simple as it is in structure, is destitute of no organ required by the nature of its economy; the animal, complex as it is in structure, is in possession of no organ which it could dispense with: from the one, nothing is withheld which is needed; to the other, nothing is given which is superfluous: in the one, there is economy without niggardliness; in the other, munificence without waste.
CHAPTER II.
Two distinct lives combined in the animal—Characters of the apparatus of the organic life—Characters of the apparatus of the animal life—Characteristic differences in the action of each—Progress of life—Progress of death.
Of the two sets of functions carried on by living beings, it has been shown, that the plant performs only one, while the animal exercises both. The two lives thus in continual play in the animal differ from each other as much as the process of vegetation differs from that of thought, yet they are united so closely, and act so harmoniously, that their existence as distinct states is not only not apparent to ordinary observation, but the very discovery of the fact is of recent date, and forms one among the splendid triumphs of modern physiology. Their action is perfect, yet their separate identity is so distinctly preserved, that each has its own apparatus and its own action, which are not only not the same, but, in many interesting circumstances, are in striking contrast to each other.
1. In general the organs that belong to the apparatus of the organic life are single, and not symmetrical; the organs that belong to the apparatus of the animal life are either double, or symmetrical, or both. As will be shown hereafter, the heart, the lungs, the stomach, the intestines, the liver, the pancreas, the spleen, the instruments by which the most important functions of the organic life are carried on, are single organs. (Chap. 5.) The figure of each is more or less irregular, so that if a line were carried through their centre, it would not divide them into two equal and precisely corresponding portions. On the contrary, the organs of the animal life are symmetrical. The brain and the spinal cord are divisible into two perfectly equal parts. (Chap. 5.) The nerves which go off from these organs for the most part go off in pairs equal in size and similar in distribution. (Ibid.) The trunk, so important an instrument of voluntary motion, when well formed, is divisible into two perfectly corresponding portions. (Ibid.) The muscular apparatus of one half of the body is the exact counterpart of that of the other; while the arms, the hands, and the lower extremities are not only double, but the organization of the one is precisely similar to that of its fellow.
2. In general, the apparatus of the organic life is placed in the interior of the body, while that of the animal life is placed on the external surface. The organic organs are the instruments by which life is maintained. There is no action of any one of them that can be suspended even for a short space of time without the inevitable extinction of life. But the animal organs are not so much instruments of life as means by which a certain relation is established between the living being and external objects. And this difference in their office is the reason of the difference in their position. Existence depending on the action of the organic organs, they are placed in the interior of the body; they are fixed firmly in their situation in order that they may not be disturbed by the movements of locomotion; they are enveloped in membranes, covered by muscles, placed under the shelter of bones, and every possible care is taken to secure them from accident and to shield them from violence. Existence not being immediately dependent on the action of the organs of the animal life, they do not need to be protected from the contact of external objects with extraordinary care, but it is necessary to the performance of their functions that they should be placed at the exterior of the body. And there they are placed, and so placed as to afford an effectual defence to the organic organs. Thus the groundwork of the animal is made the bulwark of the organic life. The muscles, the immediate agents by which voluntary motion is effected, and the bones, the fixed points and the levers by which that motion acquires the nicest precision and the most prodigious rapidity and power, are so disposed that, while the latter accomplish, in the most perfect manner, their primary and essential office in relation to the muscles, they serve a secondary but scarcely less important office in relation to the internal viscera. As we advance in our subject, we shall see that a beautiful illustration of this is afforded in the structure and action of the trunk; that the trunk is moveable; that it is composed of powerful muscles, and of firm and compact bones; and that while its movements are effected by the action of the muscles which are attached to the bones, these bones enclose a cavity, in which are placed the lungs, the heart, the great trunks of the venous system, the great trunks of the arterial system, and the main trunk of the thoracic duct, the vessel by which the digested aliment is carried into the blood. (Chap. 5.) Thus, by these strong and firm bones, together with the thick and powerful muscles that rest upon them, is formed a secure shelter for a main portion of the apparatus of the organic functions of respiration, circulation, and digestion. The bones and muscles of the thorax, themselves performing an important part in the function of respiration, afford to the lungs the chief organ of this function, composed of tender and delicate tissues, easily injured, and the slightest injury perilling life, a free and secure place to act in. The fragile part of the apparatus is defended by the osseous portion of it, the play of the latter being equally essential to the function as that of the former. In like manner the tender and delicate substance of the brain and spinal cord, the central seat of the animal life, with which all the senses are in intimate communion, is protected by bones and muscles which perform important voluntary movements while the organs of sense which put us in connexion with the external world, which render us susceptible of pleasure, and which give us notice of the approach of objects capable of exciting pain, are placed where external bodies may be brought most conveniently and completely into contact with them; and where alone they can be efficient as the sentinels of the system. For this reason, with the exception of the sense of touch, which, though placed especially at the extremities of the fingers, is also diffused over the whole external surface of the frame, all the senses have their several seats in the head, the most elevated part of the body, of an ovoid figure, capable of moving independently of the rest of the fabric, and which, being supported on a pivot, is enabled to describe at least two-thirds of a circle.
Such is the difference in the structure and position of the apparatus of the two lives, but the difference in their action is still more striking.