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Aristotle: Form, function, and comparative anatomy

1999; Wiley; Volume: 257; Issue: 2 Linguagem: Inglês

10.1002/(sici)1097-0185(19990415)257

1097-0185

K. C. Blits,

Classical Philosophy and Thought

Aristotle's writings are remarkable for their breadth no less than their depth of inquiry. They examine fundamental questions of politics, rhetoric, ethics, and poetics as well as natural science, psychology, and metaphysics, among other topics. Aristotle's studies of animals are admired for their detailed and illuminating descriptions of more than 500 kinds of mammals, birds, fish, reptiles, amphibians, cephalopods, insects, and many other invertebrates. In his animal studies, Aristotle investigates and compares their form and structure, development, physiology, behavior, and ecology in order to learn how and why they live as they do. While the vast amount of information contained in Aristotle's biological writings is by itself impressive, these works form part of a larger theoretical discussion of nature (physis). D'Arcy Thompson, author of the classic On Growth and Form, says that Aristotle did for biology what Robert Boyle did for chemistry: ". . .[he] made it a science, and won for it a place in philosophy. . ."10 What we know about Aristotle's life (384–322 B.C.) comes chiefly from Diogenes Laertius' Lives and Opinions of Eminent Philosophers, which draws on earlier sources that are mostly lost to us. Aristotle (Fig. 1) was a native of Stagira, a Greek colony on the Chalcidicean peninsula. His father, Nicomachus, was the court physician to King Amyntas II of Macedonia and was descended from several generations of physicians. Aristotle was orphaned at an early age and as a young man of 17 or 18 years went to Athens to study. There he became a student of Plato and remained at the Academy until Plato's death about 20 years later. Afterwards, he lived in Assos, an Asian Minor city not far from the Aegean island of Lesbos, and then in Macedonia, where he taught the young Alexander (later "the Great"), son of King Philip. His numerous references to sites on or near Lesbos suggest that he made many of his observations of marine animals during this time. Rembrandt van Rijn, Aristotle with a bust of Homer (1654). Reproduced with permission from The Metropolitan Museum of Art. Purchase, special contributions and funds given or bequeathed by friends of the Museum, 1961. (61.198) Aristotle eventually returned to Athens and established his own school in the Lyceum, a public space outside the city walls. Some say he was called "Peripatetic" (from peripatos, "walking") because he and his students would stroll back and forth along a promenade as they conversed. Following Alexander's death in 323 B.C., Aristotle was indicted in Athens for impiety—for being thought dangerous to the political and social well-being of the city. Aristotle, who was a resident alien and not a citizen, left rather than stand trial. Theophrastus (c.371–288 B.C.), his former student and colleague, succeeded him as head of the Lyceum. Aristotle spent his last year in Chalcis, on the island of Euboea, his mother's homeland. Aristotle was heir to a well-established, vigorous tradition in medicine and related arts, mathematics, astronomy, and natural philosophy that flourished throughout the ancient Mediterranean world. The tradition was highly pluralistic, reflecting diverse perspectives, theories, and practices, even within the same "school" of thought. The oldest surviving Greek document directly bearing on anatomy is the "Hippocratic Corpus," a collection of anonymous writings (mostly between 420–370 B.C.) on medicine and surgery. The 70 or so works are varied in both content and style and probably were written by several authors. Some are theoretical discussions on health and disease or on medicine and medical methodology; some (such as the Hippocratic Oath) concern ethical issues; while still others describe therapies and procedures or case studies. Although they often present a range of opinion on any question, the different works share the view that disease has natural causes which are intelligible and in principle treatable. In the treatise on epilepsy (called in antiquity "the sacred disease"), for example, the author says that the illness is not "any more divine or more sacred than other diseases, but has a natural cause, and its supposed divine origin is due to men's inexperience, and to their wonder at its peculiar character."9 The cause of the sacred disease, he goes on to say (from having examined the pathological condition of the brain of a goat suffering from seizures), originates in the brain. The Hippocratic writings contain many anatomical facts and observations, though no comprehensive anatomical study. "On Joints" and "On Fractures," which deal with treatments for dislocations and fractures, are noteworthy in giving what appears to be the first systematic comparison between human and other vertebrate skeletons. The collection as a whole demonstrates a solid empirical knowledge of the skeleton (including the relations of bones to one another and their natural positions), joints, ligaments, and larger surface muscles based on detailed observation, external examination, and from observations of separate bones. The writers have relatively little familiarity with internal structures beyond knowing the position of internal organs—what could be learned primarily from treating wounds. The first specifically anatomical investigation separate from a surgical or medical procedure is associated by early commentators with Alcmaeon of Croton (c. 500 B.C.), a contemporary of the famous mathematician Pythagoras. Alcmaeon is said to have dissected animals and to have discovered the optic nerve and Eustachian tubes, to have traced the main sensory nerves to the brain (which he considered the seat of sensation and intellect), and to have shown that seminal fluid did not originate in the spinal cord as was generally believed. Aristotle's inquiry about animals (including man), which is developed in several books (History of Animals, Parts of Animals, Generation of Animals, Motion of Animals, Progression of Animals, Parva Naturalia, and De Anima), gives a comprehensive and coherent account of animal lives and life, and is the first such work of its kind. Theophrastus subsequently undertook a similar project with plants, for which he is known as "the father of botany." Many regard Aristotle as the founder of comparative anatomy because his overall approach is comparative and incorporates a wealth of anatomical and morphological description, his scope is comprehensive, and his methods are rigorous and systematic. Many regard Aristotle as the founder of comparative anatomy because his overall approach is comparative and incorporates a wealth of anatomical and morphological description, his scope is comprehensive, and his methods are rigorous and systematic. The so-called smooth dogfish [which he refers to as Galeos] have their eggs between the uterus, in a similar way to the 'puppy' [Scyllium canicula or Sc. stellare] and they move along into each of the two 'horns' of the uterus: the young are produced with the umbilical cord attached to the uterus, so that as the substance of the egg gets used up the embryo's condition appears to be similar to what is found in quadrapeds. The umbilical cord, which is long, is attached to the lower part of the uterus: each one is, as it were, fastened to a cotyledon, and is attached to the embryo by the middle, where the liver is situated. If you cut the embryo open, even if it no longer contains the egg, the nourishment in it is egg-like in substance. Each embryo has a chorion and membranes of its own round it, just as in quadrapeds. . .5 Line drawing from G. Rondelet's De Piscibus Marinis (Lyons, 1554) showing a young dogfish (Mustelus laevis) still attached by the umbilical cord to its mother. Rondelet's illustration is based on Aristotle's description of this elasmobranch species (History of Animals, VI.x), which is ovoviviparous. While the structure and function of the smooth dogfish's placenta are similar to that of mammals, the placenta of the former arises from the yolk sac and of the latter from allantoic tissues. (Reproduction courtesy of University of Maryland Rare Books Library.) This, like many of Aristotle's observations, was confirmed centuries later. Johannes Muller, the 19th-Century German anatomist and physiologist, was struck by Aristotle's claim, and he conducted morphological studies of his own which showed that a few cartilaginous fishes do indeed display the structure described by Aristotle. In his biological writings, Aristotle refers to "diagrams" (diagrammata) accompanying the text and to the "dissections" (anatomata), which is thought to be a lost volume of anatomical diagrams. Aristotle may be the first to have provided illustrations for scientific treatises, though none survives. Some later investigators reconstructed Aristotle's diagrams in conjunction with their own work. Among them is the 16th-Century French naturalist Guillaume Rondelet, Professor of Anatomy at Montpellier, who included a number of drawings based on Aristotle's text in his De Piscibus Marinis, a survey of marine animals (Figs. 2,3). Rondelet's drawings of an adult sepia or cuttlefish (above) and sepia eggs (below). The figures, from De Piscibus Marinis, accompany Aristotle's descriptions in the History of Animals, IV.i and V.xviii. Aristotle notes that while all cephalopods have eight feet, cuttlefish are distinct in having two long tentacles that they use for conveying food to their mouth and for fastening themselves to rocks during stormy weather. Cuttlefish eggs, he says, are similar in appearance to large black myrtle-berries, and adhere to one another in such a manner as to resemble a bunch of grapes. (Reproduction courtesy of University of Maryland Rare Books Library.) While Aristotle uncovered with remarkable skill and insight the macro-structure and interaction of many organs, his observation of fine structure was limited by lack of optical instruments. His description of the development of a chick embryo, for example, is probably as good as is possible without a microscope, but much remains invisible even to the trained naked eye. There is no evidence that Aristotle or his predecessors performed human dissections and little, if any, that they conducted experiments in the modern sense. Aristotle's reliance on animal analogies for some human anatomy, especially internal structure, produced mixed results. It was not until the work of the Alexandrian physician Herophilus (c.330–260 B.C.) and physiologist Erasistratus (c.315–240 B.C.), who routinely dissected human cadavers as well as animals in their studies of the body, that knowledge of human anatomy received a major impetus. Aristotle's zoological inquiries aim to uncover the specific natures of the various kinds of animals–to give an account of "what" or "who" they are. All animals are the same in being "alive," but what is the same is always made manifest through difference or diversity, both in number (individuation) and in kind. "Life" means particular "lives," Aristotle suggests. Aristotle's investigation of animal life is organized around discovering and explaining what he calls differences or distinctions (diaphorai) and attributes (symbebēkota) with respect to 1) their external and internal parts (moria), 2) characters or dispositions (ēthē), 3) activities or actions (praxeis), and 4) ways of life (bioi). His study thus involves what we recognize as morphology and anatomy, physiology, reproduction and development, ethology, and ecology. Yet, because each category of difference is complex in itself and may overlap others, there is no simple method for comparing animals, even though some sort of grouping is necessary in order to discern which attributes occur together and to determine significant patterns of affinity. Among the several ways in which Aristotle divides animals are by their mode of parturition into viviparous, oviparous, and larviporous; into blooded and bloodless; and according to their habitat—for example, land- or water-dwelling. Aristotle's ways of dividing and then subdividing and associating certain groups turn out to be heuristic, an analytical tool for characterizing animal kinds in relation to one another rather than an attempt to establish a unified scheme of classification, a systematic taxonomy. Aristotle says at the beginning of the History of Animals that one kind of animal differs most from another with respect to the first of the four types of difference; namely, with respect to the "parts" from which it is composed. Explaining what he means by a "part," he initially distinguishes like parts (homoiomerē) from unlike parts (anomoiomerē); i.e., those which when divided have parts like themselves (for example, flesh or blood) as compared to those which are not made up of like parts (hands or faces). Unlike parts are "composite" in that they are composed of like ones, as a hand is made of flesh, sinews, and bones. Moreover, they are "instrumental" because they are the means by which bodily functions are carried out. Like parts, however, are compounded of a more elemental sort of material—the moist (or fluid), dry (or solid), hot, or cold. And, depending on their composition, like parts may act as the material from which unlike parts are constituted, as nutriment for other parts, or as residues of nutriments. Aristotle's account of parts as like or unlike in terms of their composition roughly corresponds to the now familiar distinction between tissue and organ. As among themselves, [birds] differ in their parts in respect of the more and less. . .some of them have long legs, some short ones. . . and similarly with the other parts. Thus, as among themselves they have few parts which differ from one to another. But compared with other animals, they differ in respect of the form of their parts. One peculiarity of the birds is that they all have feathers, whereas in other animals the parts are covered with hair, or scales, or horny plates. . . Another peculiarity of birds is the beak, an extraordinary appendage to the head. It is made of bone, and serves them instead of teeth and lips. . .7 . . .Birds' beaks also differ according to their different [ways] of life (bious). Some beaks are straight, some curved; straight if they are used simply for feeding, curved if the bird eats raw meat, because a curved beak is useful for overpowering their prey. . .Those who spend their lives in swamps and are herbivorous have broad beaks, which are useful for digging and pulling up their food and for cropping plants. . .7 The meaning of "organ" for Aristotle lies in the fact that unlike parts are instrumental. "Since every instrument (organon) is for the sake of something, and each bodily part is for the sake of something, and what they are for the sake of is an activity, it is plain that the body too as a whole is composed for the sake of a full activity."4 A part, an organ, exists for performing an action—just as a bird's beak exists for feeding. Because the capacity for action is primary, the structure of any part is properly grasped by reference to what it does. Form and function go together for Aristotle. Anatomy and physiology are integral components of the same science. And if each part is constituted the way it is in order to carry out some specific function for which it is naturally suited, then the body as a whole is formed for the sake of a manifold or full action—the interdependent, coordinated set of activities which is the animal's life. Aristotle presents the notion of full or complete activity as a community of actions, as distinguished from an aggregation or merely a collection of particulars. He supposes that individual, organized beings are primary. Animal (or plant) studies, then, are for him always specific and concrete because animals are always of this or that kind. For Aristotle, the task of biological explanation or theory is to discover the principles of organization and function responsible for the various animal kinds and their relations. This requires not only locating those differences in parts, dispositions, actions, and ways of life that serve to characterize each kind of animal, but also explaining the cause of their differences. According to one way of speaking, that out of which. . .a thing comes to be [and which persists in the thing] is called a cause; for example, the bronze and the silver . . .would be the causes respectively of a statue and a loving-cup. According to another, the form or [pattern] is a cause; this is the account (logos) of what the being is. . .Again, there is the primary source of the change or the staying unchanged: for example, the [legislator] is a cause, [or] the father is the cause of the child. . .And again, a thing may be a cause as the end (telos). That is what something is for, as health might be what a walk is for. [Why] does he walk? We answer 'To keep fit' and think that, in saying that, we have given the cause.8 For an animal, "that out of which" something comes into being—its material—are its parts and what constitutes them. The "form" of the animal is more than its shape or outward look. As the botanist Agnes Arber suggests, Aristotle's notion of form is "the inherent directiveness [of the living being] made manifest" through its complete development—through its life as a whole. Understanding form in this way involves "the description and interpretation of the entire external and internal organization of the plant [or animal], from the beginning to the end of its life-history. . .and should comprehend. . .both static and dynamic elements."1 It is striking that Aristotle invites us to the study of animals not because of its possible utility, but because every animal, no matter how lowly, is a cause of wonder. The third kind of cause—as source of motion or rest in anything that undergoes change—is sometimes called "efficient" cause. Aristotle's examples of the legislator and father, however, do not so much emphasize proximate causes preceding their effects as they do the originator of a motion—legislators are responsible for the nation going to war in the same way fathers, through the action of generation, initiate the movements that begin life. Finally, cause meant as "end" is not an extrinsic purpose, but rather, as the example of "health" indicates, it inheres in the nature of a living thing. "[W]e commonly say that 'this is for the sake of that' wherever there [appears] some end which the movement reaches if nothing stands in the way."2 Aristotle offers the development of seed to maturity as a salient example: growth arises from an inherent tendency or power (dunamis) of the seed itself to reach adulthood, and it will complete its development if not prevented from doing so. The "end" of a living being, then, is the full development of its specific form or nature. Form and end converge in an animal's living its life of manifold action, from the beginning to the end of its life-history. . . .even in the study of animals unattractive to the senses, the nature that fashioned them offers immeasurable pleasures in the same way to those who can learn the causes (aitias) and are naturally lovers of wisdom (philosophois). . .Therefore we must avoid a childish distaste for examining the less valued animals. For in all natural things there is something wonderful.3 It is striking that Aristotle invites us to the study of animals not because of its possible utility, but because every animal, no matter how lowly, is a cause of wonder. Aristotle's characterization of natural things as "wonderful" suggests something akin to his statement that "philosophy begins in wonder."6 Animals are wonderful because they are distinguished essentially by self-directiveness, by their intrinsic capacity as self-organizing, self-maintaining beings to stay themselves over a single lifetime and for countless generations—which is to say, by their having an intrinsic end (telos). Whatever is wonderful is somehow a cause of inexhaustible reflection and, therefore pleasure, for human beings; it arouses our inherent desire to know the fundamental things. Dr. Blits has been a faculty member of St. John's College in Annapolis, Maryland since 1990, where she teaches, among other things, biology and philosophy. she received her Ph.D. in marine biology and biochemistry from the University of Delaware.