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large organ is, ceteris paribus, made up of a greater number of integrant parts than a small one; and if, in the small one, each of these parts is equivalent in force to a given quantity, it necessarily follows, all other circumstances being equal, that the force of the large will exceed that of the small organ by the united quantity of all the additional individual parts; and no way of escaping this conclusion can be got, except the unfair one of keeping the ceteris paribus out of view. If this were not the case, we would as soon expect to see a breach effected in the massive walls of a fortification, by a rapid and sustained discharge of musketry, as under the reiterated blows of heavy artillery. But it is the animal kingdom which chiefly concerns our present argument, and to it we therefore proceed.
It will scarcely be disputed, that the strength of the bones is always, ceteris paribus, proportioned to their size; and the slightest consideration will satisfy any one that the same principle applies equally to the muscular system, which, indeed, its structure sufficiently demonstrates. Muscles are composed of a great number of nearly parallel fleshy fibres, each equal in itself to a given force. If then the bulk of the muscles is increased either by a greater thickness, or by an additional number of such fibres, it is physically certain, even a priori, that the amount of force which they are capable of exerting will be increased in exact proportion. The great increase of muscular power consequent upon the increased size of the muscular fibres in the arm of a blacksmith, or in the leg of an operadancer, is a familiar example of the one, as the powerful vigour of the singularly muscular dray, contrasted with the slender, though active, frame of the race horse is of the other. The same principle is familiarly recognized in the fable of the old man showing his sons how easy it is to snap asunder any individual-stick of a bundle, but how difficult to overcome their united powers of resistance.
Although muscular, like cerebral size, is, ceteris paribus, a measure of power, still it is by no means the only condition. There are circumstances in which great muscular
energy is required in combination with as small muscular bulk as is possible to be attained, and there are others where bulk is of no consequence. To effect this modification, a beautiful arrangement is made by Nature, and in strict accordance with the principle we are now proving.
Motion is the result of muscular contraction, but muscular contraction takes place only in consequence of the stimulus of the will conveyed by the nerve, whose ramifications form a part of the muscle itself. Hence strong contraction may arise either from large muscle and moderate stimulus, or from moderate muscles and strong stimulus. Thus, in fishes, which live and move in a medium almost in equilibrium with their own bodies, and which, of course, require no active effort to support themselves in a position different from that given by the ordinary laws of gravitation, and in which, consequently, increased balk is attended with no material disadvantages, the power of motion depends in a high degree on the great size of muscle, and in a small degree on nervous excitement or size of nerve. But in men and other animals, who require a constant effort to preserve their upright position, and in whom increase of muscular bulk would add directly to the sum of the difficul. ty, by adding to the weight, the same end is accomplished by an increase in the supply of nervous excitement, or, in other words, by a relatively much larger nerve in proportion to the muscle which it is destined to supply. In birds, again, where the disproportion between their own gravity and that of the air is so strikingly great, and where, consequently, every additional muscular fibre would but add, by its weight, to the difficulty of rising from the ground, the same relative increase of nerve over muscle is carried to a still greater degree. But in this arrangement, the law of Size is still in force; for, in all of these instances, wherever the supply of nervous energy and other conditions, are found to be equal, there Size of muscle invariably indicates the degree of power. Had the power of motion in birds depended on Size of muscle alone, and these organs been proportionally as large as in fishes, they must of necessity either have remained
for ever chained to the surface of the earth like man, or they must have perished, from absolute inability to fill the place which Nature had destined for them in the scale of creation; and Size in one part of the organ has thus been given to obviate the disadvantages which it would have entailed had it been possessed by the other.
It may be objected, that the biggest men are not always the most powerful in bodily strength; and that the maniac, or an individual in the delirium of fever, is often able to overcome the united efforts of several persons to restrain him. But this is still in strict accordance with our principle; for the other conditions are not the same ; and it is quite certain, that if they were, if a big muscular man, for instance, were subjected to the same morbid excitement as the smaller, he would display an energy of motion greater than the other, in exact proportion to the greater size of his muscles; and it is no exception to this, to produce a bulky individual of a weak lymphatic constitution, made up of water and fat, rather than of muscle, and to say that he has less bodily energy than another individual of smaller size, but of a bilious and firm habit, and in whom the muscular system is at its highest state of perfection. The very contrasting of such individuals, without attending to the ceteris paribus, is a total departure from the principle which we are advocating, and, consequently, need not now occupy our attention.
That the law of Size holds in regard to the blood vessels and heart is self-evident to every one who knows that a tube of three inches diameter will transmit more water than a tube of only one inch. And the same may be said in regard to the lungs, liver, kidneys, and every other part. If a liver, suppose of four square inches, can secrete four ounces of bile, it is perfectly manifest, that one of eight square inches will be able, all other things being equal, to secrete a quantity greater in proportion to its greater size. If this law did not hold true, what would be the advantage of large and capacious over small and confined lungs? There could be none.
In regard to the nerves or organs of sensation also, which
in function and structure approximate more closely to the brain, the disputed proposition of Size being a measure, or element of
power, is easily demonstrable. Speaking generally, there are two classes of nerves distributed over the body, those of motion and those of sensation or feeling. In accordance with our principle, the nerves of motion are, in most instances, greatly smaller than those of sensation, and for this reason ; in producing motion, the muscle is the essential or chief apparatus, and the nerve is required only to communicate to it the impulse or stimulus; but in sensation the reverse is the case, the nerve itself is the chief instrument, and the part on which it is ramified is merely a medium for putting it in relation with the specific qualities which it is destined to recognize. Thus the eyeball is merely an instrument constructed in accordance with the laws of light, by means of which the proper impression may be made on the optic nerve, and thence transmitted to the mind. In accordance with this principle of Size being a chief element in power, we find that the olfactory, the optic, and other nerves of sensation, have a constant and often enormous excess of volume over the muscular nerves, or those of motion, in the same animal ; and that as a general law, the nerves of sensation are always proportioned in size to the extent to which sensation is possessed. It is stated by a late very able writer,* that in the spinal nerves of man, for example, the dorsal roots, or those belonging to sensation in the nerves supplying the arm, have at once an excess of volume and of surface at least five times greater, both for each individual fibre, and for the bundle resulting from them, than the anterior roots, or those belonging to motion. And the rationale of this is evident; for it is in the hand that the greatest power of touch resides, and it is by these nerves that the hand is supplied.
Another fact, mentioned by the same author, shows clearly the universality of the disputed principle. The roots of sen
• Desmoulins, Anatomie des Systèmes Nerveux des Animaux à vertèbres,
sation in the spinal nerves going to the arm are about five times larger than the corresponding roots at other parts of the spinal cord, which, it must be observed, are distributed to parts where touch is imperfectly possessed. He adds, that, comparing the size or mass of each kind of nerve with the extent of skin and muscle on which each is ramified, the nerve of sensation will be found in the mammalia often more than a hundred times more voluminous than that of motion ; and that, allowing for the greater thickness of muscle, this disproportion will be enormously increased. And as an instance, he mentions that the single nerve of feeling ramified, on the tactile extremity of the proboscis of the elephant, exceeds in size the united volume of all the muscular nerves of that organ.
Having just shown, that in animals possessed of acute sensation, the corresponding nerves greatly predominate over those of motion, I may now add, in corroboration of our principle, that in other animals, in which muscular power greatly predominates over feeling, the balance between the nerves becomes changed. In the horse and the ox, for example, which, from the nature of their covering, have very imperfect touch, with great bodily strength, the sum total of the muscular roots in the nerves going to the four limbs exceeds, by at least one-third, that of the sensitive roots, where in men the proportions were inversely as five or six to one. In like manner, in birds and reptiles with scaly skins and limited touch, the same preponderance of the nerve of motion over that of sensation obtains. And what is curious enough, wherever nature has given a higher degree of sensation or touch to any particular part, there the corresponding nerve is invariably increased. This is observed, for example, in the nerves of the tail in some species of monkeys, in those of the wings in some bats, and in those of the claws of some species of birds, and the increased Size is confined exclusively to the part possessed of the increased function.
We come now to consider particular modifications of sen