case from his model; he endeavours to surpass it, and he is right. To make this understood we cannot do better than quote a passage in which L. Foucault compares the screwpropeller of ships to the organs of swimming in fishes :

"In our machines," said he,*"we have usually a great number of parts entirely distinct one from the other, which only touch each other at certain points; in an animal, on the contrary, all the parts adhere together; there is a connection of tissue between any two given parts of the body. This is rendered necessary by the function of nutrition which is continually going on, a function to which every living being is subject during the whole of its existence. We can, besides, understand the absolute impossibility of obtaining a continued movement of rotation of one part on another, while still preserving the continuity of these two parts."

Thus, a profound difference separates mechanisms employed by nature from those invented by man; the former are subject to special requirement from which the latter can be freed. The muscle can only act under the condition of being attached by its vessels and nerves to the rest of the organism. No portion of the body, not even the bones themselves, which have the least vitality, can be free from this necessity.

One might find, in the animal organism, many other mechanical appliances, the arrangement of which resembles that of machines invented by man, but with differences ever of the same kind as those which we have just described.

For instance, the circulation of the blood is effected in living beings by a veritable hydraulic machine, with its pump, valves, and pipes. But the fundamental difference between this complicated mechanism and machines constructed by man, arises from the absence of independent portions, and especially of the piston. The heart is a pump without a piston, and its variations of capacity are obtained by the contractility of the coats of the vessels themselves. With the exception of this difference, we find perfect analogies between the circulatory apparatus of animals and hydraulic motive powers. The function of the valves is identical in both in spite of apparent differences.

"Journal des Débats," Oct. 22, 1845.

We have formerly noticed in the circulation of the blood an influence which regulates and increases the effective work of the cardiac pump; it depends on the elasticity of the arteries.* In like manner, in hydraulic machines, man has recourse to the employment of elastic reservoirs, to utilize more fully the work of pumps, and to render uniform the movement of the liquid, notwithstanding the intermittent character of the motive This effect may be compared to that which we have before remarked in the elasticity of muscles.

power.

Dynamic energy of animated motors.-Animated motive powers and machines are subject to the same estimation of work; it is the dynamic energy of the former as compared with the latter.

The production of external work corresponding to 75 kilogrammetres per second, has been called the horse-power, or, in more general terms, the motive power of one horse, it being supposed that one horse could develop the same amount of work.

But animal motors cannot work incessantly, so that the horse-power would represent at the end of the day a much greater amount of work than the animal could have produced, had it been employed as a motive force.

Man is estimated much lower as to his dynamic energy, (of a horse-power), and yet, if we only require from the muscular force of a man an effort of short duration, it will furnish dynamic energy exceeding that of a horse-power. In fact, the weight of a man is often more than 75 kilogrammes ; each time that the body is raised to the height of a metre per second, in mounting a staircase, the man has effected during this second the work adequate to one horse-power. And if, during several instants, he can give to his ascent the speed of two metres per second, this man will have developed the work of two horse-power.

Thus, in our estimate of the work done by the greatest or the smallest animals, we must consider it as a multiple or a fraction of the ordinary measure of horse-power.

* "Physiologie médicale de la circulation du sang."

CHAPTER VIII.

HARMONY BETWEEN THE ORGAN AND THE FUNCTION.DEVELOPMENT HYPOTHESIS.

Each muscle of the body presents, in its form, a perfect harmony with the nature of the acts which it has to perform-A similar muscle, in different species of animals, presents differences of form, if the function which it has to fulfil in these different species is not the same-Variety of pectoral muscles in birds, according to their manner of flight-Variety of muscles of the thigh in mammals, according to their mode of locomotion-Was this harmony pre-established — Development hypothesis-Lamarck and Darwin.

THE comparison between ordinary machines and animated motive powers will not have been made in vain, if it has shown that strict relations exist between the form of the organs and the characters of their functions; that this correspondence is regulated by the ordinary laws of mechanics, so that when we see the muscular and bony structure of an animal, we may deduce from their form all the characters of the functions which they possess.

It is known that the transverse volume of a muscle corresponds with the energy of its action; that the athlete, for instance, is recognized by the remarkable relief in which each of his muscles stands out under the skin. But less is known concerning the physiological signification of the length of the muscles, that is to say, the less or greater length of their contractile fibres. And yet Borelli has already given the true explanation. In his opinion, as we have seen, this length of red fibre is proportioned to the extent of movement which the muscle is fitted to produce.

This distinction between the contractile or red fibre and the inert fibre of the tendon is of the utmost importance. Experiment has shown that the muscles when they contract are shortened to an extent which represents a constant fraction of their length. We may, without erring from the truth, estimate at of their length, the extent to which a muscle

can contract. But, whatever may be the absolute value of this contraction, it is always in proportion to the length of red fibre; that is the result of the nature of the phenomena which produce work in the muscle.

Thus, every muscle whose two points of attachment are susceptible of being much displaced by the effect of contraction, must necessarily be a long muscle. On the contrary, every muscle which has to produce a movement of short extent must of necessity be a short muscle, whatever may be the distance which separates the two points of attachment. Thus, the flexors of the fingers and toes are short muscles; but they are furnished with long tendons, which convey even to the phalanges of the fingers or toes the slight movement originated at a considerable distance at the fore-arm or the leg.

It is easy to estimate, in the dead body, the extent of the displacement which a muscle can exercise on its two points of attachment. By producing the movements of flexion or extension in a limb, we can ascertain with sufficient exactness the extent by which they separate or draw together the osseous attachments of its muscles. In a recent skeleton we can also judge with sufficient accuracy of the amount of these movements by the extent to which the articulated surfaces can glide over each other.

In examining the muscular frame of man we are struck with the extreme length of the sartorius muscle; it is easy to be seen that no other can displace to such an extent its points of bony attachment. The sterno-mastoidal and the magnus rectus abdominis are, after this, the longest muscles these also are muscles which have very extensive movements. We might thus cause all the muscles of the organism to pass under review, and in them all we should see that the length of the red fibres corresponds with the extent of the movement which this muscle has to execute. But, in the study, we must be on our guard against a cause of error which would tend to arrange certain short muscles among those which are longer.

Borelli himself has noticed this cause of error; he has shown how penniform muscles, that is to say, those whose fibres are inserted obliquely into the tendon, like the barbs of

a feather into the common shaft, are short muscles which appear like long ones. These considerations are indispensable when we wish to understand the action of the various muscles of the organism; it is only by this means that we can estimate the real length of their contractile parts. Though the harmony between the form and the function of different muscles is revealed everywhere in the anatomy of the human frame, this harmony becomes much more striking if we compare with each other different species of animals. Comparative anatomy shows us, in species closely allied to each other, a singular difference in the form of certain muscles whenever the function of these muscles varies. Thus, in the kangaroo, essentially a leaping animal, we find an enormous development of the muscles of leaping, the glutei, the triceps extensor cruris, and the gastrocnemial muscles.

In birds the function of flight is exercised under very different conditions in different species; so, also, the anatomical arrangement of the muscles which move the wing, the pectoral muscles, varies in a very decided manner in different species. To show the perfect harmony which exists between the function and the organ, it would be necessary to enter into long details of the mechanism of flight. The reader will find, We will content our

farther on, explanations on this head. selves with giving in a few words the differences which have been observed in the movements of the wing, and in the form of the muscles which produce them.

Every one has remarked that birds which have a large surface of wing, as the eagle, the sea-swallow, &c., give strokes of only a slight extent; that depends on the great resistance which a wing of so large a surface meets with in the air.

Birds, on the contrary, which have but very little wings, move them to a great extent, and thus compensate for the slight resistance which they meet with from the air; the guillemot and the pigeon belong to the second group. If it be admitted that the first-mentioned birds must make energetic but restricted movements, and that the second must move with less energy, but with greater amplitude of stroke, the conclusion arrived at must necessarily be that the first ought to have large and short pectoral muscles, while in the