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ON
THE NATURE OF LIMBS.
THE chief difficulty that I have encountered in the endeavour to fulfil the request with which I have been honoured by the Managers of this Institution, is one which the able Secretary will be surprised to hear I owe to him: it has been the attempt to give a plain answer to Mr. Barlow's question, ‘By what title shall I announce your Lecture?’
It was not until I had written and erased two or three which first suggested themselves that I became fully conscious how foreign to our English philosophy were those ideas or trains of thought concerned in the discovery of the anatomical truths, one of which I propose to explain on the present occasion in reference to the limbs or locomotive extremities.
A German anatomist, addressing an audience of his countrymen, would feel none of the difficulty which I experienced. His language, rich in the precise expressions of philosophic abstractions, would instantly supply him with the word for the idea he meant to convey; and that word would be ‘Bedeutung.’ It is the ‘Bedeutung’ of the limbs which is my present subject; and the literal translation of the word is ‘signification.’
I had written at first, ‘On the Signification of the Limbs of Man and Animals;’ when it occurred to me that a practical audience might deem that there was small need of a learned lecture to prove that matter, and might be disposed to think that any old Pensioner at Greenwich or Chelsea could certify better than a Professor, that ‘arms and legs’ signified a good deal.
Our word ‘meaning’ as applied to what I shall endeavour to prove, in regard to those useful appendages, would convey as false or feeble an idea of my meaning.
‘On the idea of the limbs’ might be understood only by those who knew that the word was used in the sense it bears in the Platonic philosophy.
‘Homology’ seems now to be accepted as the name of that study or doctrine the subject of which is the relations of the parts of animal bodies understood by the German word ‘Bedeutung;’ and in the technical language of anatomical science I should define the present lecture as being: “On the General and Serial Homologies of the Locomotive Extremities.” But such a title would have been comprehended only by anatomists, and I knew that your Secretary had in view the information of a more general audience, to whom such technical phraseology would be less intelligible perhaps than the literal translation of the German term.
The ‘Bedeutung,’ or signification of a part in an animal body, may be explained as the essential nature of such part—as being that essentiality which it retains under every modification of size and form, and for whatever office such modifications may adapt it. I have used therefore the word ‘Nature’ in the sense of the German ‘Bedeutung,’ as signifying that essential character of a part which belongs to it in its relation to a predetermined pattern, answering to the ‘idea’ of the Archetypal World in the Platonic cosmogony, which archetype or primal pattern is the basis supporting all the modifications of such part for specific powers and actions in all animals possessing it, and to which archetypal form we come, in the course of our comparison of those modifications, finally to reduce their subject.
The ‘limbs’ to which the limits of the present Discourse confine its application, are those of the Vertebrate Series of animals; they are the parts called the ‘arms’ and ‘legs’ in Man; the ‘fore-’ and ‘hind-legs’ of Beasts ; the ‘wings’ and ‘legs’ of Bats and Birds ; the ‘pectoral fins’ and ‘ventral fins’ of Fishes. I take for granted that it is generally known, as it is universally admitted by competent anatomists and naturalists, that these limbs or locomotive members, which, according to their speciality of form, have received the above special names, are answerable or ‘homologous’ parts : that the arm of the Man is the fore-leg of the Beast, the wing of the Bird, and the pectoral fin of the Fish. This special homology has been long discerned and accepted; but the general homology of the parts or their relation to the vertebrate Archetype, in short their ‘Bedeutung’ or essential nature, is not generally known. Some of the keenest wits and deepest thinkers amongst the anatomists of the German Philosophical School have endeavoured to penetrate the mystery, and have propounded the views which have resulted from such their attempts. But those views have never received the sanction of even partial assent; and, if the conclusion to which I have arrived be the correct one, such assent has not been unreasonably withheld.
It must be owned, however, that the non-acceptance of these generalizations has been due more to indifference and to the non-appreciation of the value of the inquiries, than to a rigorous investigation of their merits. Very few of the exact conclusions as to the general homology of parts of the skeleton of animals have been admitted with thorough comprehension and fruition of the discovery by the actual cultivators of Natural History in this country; so that I can scarcely appeal to an example in illustration of my meaning with any hope that it will prove such to more than a very small portion of my hearers.
Some however may understand and assent to the proposition that “the basilar part or process of the occipital bone in human anatomy is the ‘centrum’ or body of a cranial vertebra.” Now, by virtue of this truth, mark what the human anatomist comprehends! First, that the ‘pars basilaris’ is not a process in the proper signification of the term, but a self-subsistent, independent element of the skull; whereby he is prepared for its primitive appearance as a distinct part in the embryo, and as a persistent distinct ‘bone’ in all the cold-blooded Vertebrata. He further recognises it to be a member of the same series of bones as the flat bodies of the cervical vertebræ, the thick bodies of the dorsal and lumbar vertebræ, and the broad bodies of the sacral vertebræ; and he understands why it differs from the other ‘processes of the occipital bone’ by its primitive relation to the embryonic ‘chorda dorsails.’ All this, and much more comfortable knowledge of the ‘processus seu pars basilaris ossis occipitalis,’ is implied by the definition of its general homology, i. e. of its essential nature, signification, or ‘Bedeutung’; and it is precisely the same kind of knowledge which I imply by the word ‘nature’ in reference to the limbs.
The parts to which I here refer, and to which alone the reasoning will apply that leads to the desired conclusion in the present lecture, are those in the Vertebrated animals, serving chiefly for locomotion, but sometimes adapted to other offices. Many and multiform parts answering these purposes are present in the Invertebrated animals; but their framework is formed out of a distinct system of hard parts from those employed in the Vertebrata. Here it is the internal or endo-skeleton : in the Invertebrata it is the hardened skin, the dermo- or exo-skeleton.
The hard parts of the leg of a Crab or an Insect may be ‘analogous’ to the bones of the limb of a Quadruped, but they are not ‘homologous’ with them; and where there is no special homology, there can be no relations of a higher or more general homology between the parts*.
The Vertebrated animals enjoy as extensive and diversified a sphere of active existence as the In vertebrated. They people the seas, and can move swiftly both beneath and upon the surface of water: they can course over the dry land, and traverse the substance of the earth: they can rise above that surface and soar in the lofty regions of aërial space.
The instruments for effecting these different kinds of locomotion—-diving and swimming, burrowing and running, climbing and flying—are accordingly very different in their configuration and proportions. The simplest form of the locomotive member is that of the fin. The marine mammal called Dugong here offers us an example of such (fig. 1). It is a strong, stiff, short, broad, flat, and obtusely pointed paddle or oar; without other apparent joint than that which unites it to the body it has to propel: a joint permitting that degree of rotation with the oblique stroke that makes the movement of the oar most effective.
The instrument for burrowing, such as the Mole presents (fig. 2), is not very different in form and character from the fin; but being destined to displace a denser element than water, it is shorter in proportion to its breadth, and much stronger: it resembles the fin in consisting, seemingly, of but one segment or joint, and being moveable as a whole only where it is set on to the trunk. The free border, however, instead of being smooth and thin, is notched, and armed with a row of hard, tooth-like, horny points, adapted for scraping and throwing back the soil. With such rapidity does the mole effect this purpose*, that it may literally be said to ‘swim through the earth.’
The third form of limb or locomotive member here exhibited (fig. 3), offers a striking contrast to the burrowing trowel we were last contemplating. It is a thin, vastly expanded sheet of membrane, sustained, like an umbrella, by slender rays, and flapped by means of these to and fro in the air; and with such force and rapidity, as, combined with its extensive surface, to make it react upon the attenuated element more powerfully than gravitation can attract the weight to which the limb is attached, and consequently the body is raised aloft and moved swiftly through the air ; in brief, the animal flies, and these instruments of its aërial course are called ‘wings.’
When a quadruped has to move swiftly along the surface of the earth by reacting upon the hard ground, its limbs are as remarkable for their length and slenderness as those of the burrower or swimmer are for their shortness and breadth. In the racer the instruments of its rapid course are four long tapering columns, with joints permitting them to bend in opposite directions, and of the form represented in the diagram (fig. 4) and familiar to all: each column rests upon a slightly expanded base encased by the hard horny sheath which we call the ‘hoof.’
When the limbs are adapted for grasping as well as running, they are divided at their extremities into moveable appendages or digits, one of which can be opposed to the others and retain the object of their mutual pressure. Each of the four extremities is so organized in the ape and monkey, which are thus especially adapted for climbing and living in trees.
In Man the principle of special adaptation goes further; and, whilst one pair of limbs is expressly organized for locomotion and standing in the erect position, the other pair is left free to execute the manifold behests of his rational and inventive Will, and is exquisitely organized for delicate touch and prehension, emphatically called ‘manipulation.’
Such are some of the more striking amongst the countless purposes to which the parts of animals called ‘limbs ‘are adapted, and such the consequent diversity of their outward shapes and proportions. We cannot be surprised at this; it could not be otherwise: the instrument must be equal to its office. And consider the various devices that human ingenuity has conceived and human skill and perseverance have put into practice in order to obtain corresponding results!
To break his ocean-bounds the islander fabricates his craft, and glides over the water by means of the oar, the sail, or the paddle-wheel. To quit the dull earth Man inflates the balloon, and soars aloft, and, perhaps, endeavours to steer or guide his course by the action of broad expanded sheets, like wings. With the arched shield and the spade or pick he bores the tunnel: and his modes of accelerating his speed in moving over the surface of the ground are many and various. But by whatever means or instruments Man aids, or supersedes, his natural locomotive organs, such instruments are adapted expressly and immediately to the end proposed. He does not fetter himself by the trammels of any common type of locomotive instrument, and increase his pains by having to adjust the parts and compensate their proportions, so as best to perform the end required without deviating from the pattern previously laid down for all. There is no community of plan or structure between the boat and the balloon, between Stephenson's locomotive engine and Brunel's tunnelling machinery: a very remote analogy, if any, can be traced between the instruments devised by man to travel in the air and on the sea, through the earth or along its surface.
Nor should we anticipate, if animated in our researches by the quest of final causes in the belief that they were the sole governing principle of organization, a much greater amount of conformity in the construction of the natural instruments by means of which those different elements are traversed by different animals. The teleologist would rather expect to find the same direct and purposive adaptation of the limb to its office as in the machine. A deep and pregnant principle in philosophy, therefore, is concerned in the issue of such dissections, and to these, therefore, I now pass, premising that the end in view will be attained without extending the comparison beyond the framework of the limbs, or the leverage of the bones and joints.
The human anatomist is of course familiar with this part of the skeleton in man. The arm is suspended from a broad bone called ‘scapula’ (Frontispiece, and Plate I. fig. 6, 51), and the shoulder-joint is strengthened by a slender bone called ‘clavicle’ (58), which abuts against the top of the ‘sternum’ (59), forming with its fellow an inverted arch, called in Comparative Anatomy the ‘scapular arch.’ The arm proper is appended to this arch : its first joint or segment is formed by a single long bone, the ‘humerus’ (53); its second joint, by a pair of shorter and more slender bones, the ‘radius (55) and ulna’ (54) ; and the hand or third segment is formed by a group of little thick bones, the ‘carpals’ (56), and by five rays or digits; one (I) consisting of three segments, the rest (II—v) of four segments each; the five bones joining the carpus being called ‘metacarpals,’ and the others the ‘phalanges.’
When we proceed to compare with this the skeleton of the corresponding limb of the horse, ox, or other hoofed animal, the simplification of structure seems not to be in the ratio of the loss of function : almost all that the hoof can be made to do is to rest upon or beat against the ground; and yet we find in the solidungulate limb (fig. 4) the broad scapula (51), the long humerus (53), the radius (55) and ulna (54), the carpus (56), the metacarpus (57) and the digital phalanges. There is a diminution and simplification of accessory parts, but the essentials are maintained : it is obvious that the same type has governed the formation of the two limbs compared. The most marked distinction is the total absence of the clavicle in the hoofed quadruped : the shoulder-joint did not need to be made the fixed point upon which the fore-limb might rotate in a variety of directions ; on the contrary, the movements of the humerus in th...
