Comparative Anatomy

Comparative anatomy deals with the comparative study of animal organs and establishes their morphological similarity, based on the commonality of their origin (homology). Thus, comparative anatomy makes it possible to establish the historical character (phylogeny) of the kinship ties of various animals, and of man. Animal anatomy, or zootomy, was studied as early as antiquity. Aristotle, when describing animals, pays much attention to anatomy, emphasizing the importance of the comparative method, since the little-known is explained by comparison with the better-studied. In the Middle Ages, the method of comparison was revived in part only in the anatomical works of P. Belon (Pierre Belon, 1517–64) on birds, in the embryological studies of Fabricius (Girolamo Fabrizio ab Aquapendente, 1537–1619), and in the anatomical work of Severino (Marco Aurelio Severino, 1580–1656), "Zootomia Democritea." The founder of comparative anatomy can be considered a collaborator of Buffon (Georges de Buffon, 1707–88), L. Daubenton (Louis Daubenton, 1716–1800), who processed the anatomical part of Buffon's work "Histoire naturelle," covering the anatomy of man, mammals, and birds. He describes all organ systems and especially the skeleton, which is compared in detail among various animals and with the human skeleton. Interesting comparative anatomical studies, especially of the bird skeleton and the auditory organ of vertebrates, also belong to the famous Dutch anatomist P. Camper (Petrus Camper, 1722–89). Even more consistently than in Daubenton's work, the method of comparison was carried out in the works of F. Vicq d'Azyr (Félix Vicq-d'Azyr, 1748–94), who worked especially on the limbs and dental system of mammals. Among English scientists, J. Hunter (John Hunter, 1728–1793), the founder of the first comparative anatomical museum, deserves mention. In Germany, the study of comparative anatomy was introduced by J. Blumenbach (Johann Blumenbach, 1752–1840), who worked mainly on the skeleton of mammals. All these researchers, however, always proceeded from human anatomy when making comparisons. E. Geoffroy Saint-Hilaire (Étienne Geoffroy Saint-Hilaire, 1772–1844) significantly enriched comparative anatomy. When comparing the organs of various animals, he came to establish a special category of similarity, which he called "analogy" (now "homology") and defined by the constancy of relationships with other organs, rather than by function or form, which turn out to be highly variable. Also interesting are his "principle of the balancing of organs" (principe de balancement des organes), his considerations on progressive and regressive development, on the significance of rudimentary organs, thoughts on the unity in the organization of various types of animals, and finally the comparison of embryos of higher vertebrates with adult lower ones, i.e., a whole series of new thoughts connected with the idea of the mutability of species, which he, just like J. Lamarck, vainly tried to introduce into science. Unlike all his contemporaries, Saint-Hilaire, when speaking about the connection between an organ and its function, very successfully avoided a teleological explanation of the structure of organs. Despite all these studies, it was nevertheless only G. Cuvier (Georges Cuvier, 1769–1832) who managed to raise comparative anatomy to the level of an independent discipline. Cuvier proceeds from the study of the anatomy of lower marine animals—radiates, worms, and mollusks. Among vertebrates, he was the first to study the anatomy of fish in detail, but he did especially much on the anatomy of mammals, and for the first time, both the skeletons of fossil animals were studied. Basing his generalizations exclusively on strictly established facts, Cuvier refrained from the lightweight natural-philosophical speculations of that time; however, at the same time, he could not rise to the height of those ideas that were already germinating at that time in the free minds of Saint-Hilaire, J. Lamarck, and others, but did not yet have a sufficient scientific basis. Cuvier was convinced of the immutability of species and divided the entire animal kingdom into several types, each built according to a special plan, completely excluding the possibility of comparison between animals of different types. This division into types represented a great success for systematics and, in a modified form, has survived to the present time. However, Cuvier's most remarkable achievement is his theory of correlations, according to which every organism is an integral system, not one of the parts of which can change without causing a change in the others. During the further development of comparative anatomy, the doctrine of correlations was unfortunately not developed further. The evolutionary doctrine and the phylogenetics associated with it distracted attention from this direction for a long time, and only at the present time do glimpses of its revival appear in "biological" or "synthetic" morphology. As factual material accumulated, the need for broader generalizations naturally grew. The romantic natural philosophy of the early 19th century also exerted its influence in the field of comparative anatomy, especially in Germany, in particular on the works of the famous poet and naturalist Goethe (Johann Wolfgang Goethe, 1749–1832), who wrote a number of comparative anatomical studies and, among other things, an article on the composition of the skull from 6 vertebrae. Goethe's fantastic images were of no great importance for the development of scientific comparative anatomy. The idea of the composition of the skull from several vertebrae was expressed 10 years earlier by another natural philosopher, the zoologist L. Oken (Lorenz Oken, 1779–1851). The idealistic direction in morphology persisted until the middle of the 19th century. The vertebral theory of the skull was accepted by the scientific world and developed further. The types of the animal kingdom were viewed as built according to a definite plan, representing a more or less perfect embodiment of the creator's idea. The vertebral theory of the skull received its highest development in the theory of the "archetype" of R. Owen (Richard Owen, 1804–92), the most prominent comparative anatomist of England, who introduced into science a number of basic concepts that are still the foundation of comparative anatomy today. Thus, R. Owen distinguishes two categories of similarity between the organs of various animals: analogy and homology. He calls organs of the same function analogous; homologous are the corresponding organs in various animals, regardless of changes in their form and function. Along with these major achievements, R. Owen nevertheless remained under the sway of the outdated idealistic direction and developed in detail the doctrine of an archetype common to all vertebrates. He firmly held to the doctrine of the independence and immutability of types and met the doctrine of C. Darwin with extreme hostility. However, precise research replaced natural philosophy, represented by J. Müller (Johannes Müller, 1801–58), who investigated the anatomy of lower vertebrates (especially hagfish), which was of enormous importance in the history of comparative anatomy, and a histological element was also introduced into the research. Simultaneously with the study of the anatomy of adult animals, comparative embryology also developed. This direction received a strictly scientific basis, starting with the famous studies of K. Baer (Karl Ernst von Baer, 1792–1876), in which, besides precise observations, there are many remarkable conclusions. Among other things, K. Baer compares the embryonic notochord of a chicken with the spinal cord of cartilaginous fish and gives a correct interpretation of the significance of the gill slits and arches of the embryo, found by M. Rathke (Martin Rathke, 1793–1860), who discovered gill slits and gill circulation in the embryos of birds and mammals, the change of excretory organs, and paid special attention, among other things, to the phenomena of regressive development. The most powerful development of comparative anatomy began with the triumph of the evolutionary idea. C. Darwin himself did not possess sufficiently deep knowledge in the field of comparative anatomy and therefore did not by any means use the factual material that had already been accumulated at that time. The most energetic proponent of C. Darwin's ideas in England was Huxley (Thomas Huxley, 1825–95), who earned special fame for his comparative anatomical study of the skull, where he breaks down the vertebral theory of the skull, and consequently the theory of archetypes of R. Owen. Idealistic morphology had outlived its time; it was finally discredited after the blows dealt to it by T. Huxley; the path for the penetration of the evolutionary idea into comparative anatomy was cleared. The apostle of Darwinism in Germany was E. Haeckel (Ernst Haeckel, 1834–1919), who also exerted a certain influence on the further development of comparative anatomy by constructing the genealogical trees of various organisms, which laid the foundation for the phylogenetic direction, and by popularizing the "biogenetic law," which had already been finally formulated by F. Müller (Fritz Müller, 1821–97) as the rule of the brief repetition by each higher organism during its individual development ("ontogeny," in Haeckel's terminology) of those stages that its ancestors passed through in their historical development ("phylogeny"). The application of the biogenetic law gave support to the embryological method in comparative anatomy. The evolutionary idea breathed new life into comparative anatomy and quickly brought it to an unprecedented flourishing. The concept of homology received a different definition. K. Gegenbaur (Gegenbaur) gives the following formulation: "By homology (in the strict sense of the word) is denoted the relationship between two organs of the same origin, which consequently developed from the same primordium and exhibit the same morphological relationship."

Establishing homology as the main task of Comparative Anatomy now becomes a means for determining the kinship relations between organisms, i.e., for establishing their phylogeny. Homologous organs of various animals can exhibit a very different degree of perfection, which, from the point of view of evolutionary theory, is explained by their progressive or regressive development. Progressive development is accompanied by the complication or differentiation of an organ or system of organs, which is connected with the division of labor occurring here between individual parts of the organ, which take upon themselves the performance of various specific functions. During regressive development, the structure of the organ is simplified and its dimensions are reduced up to the disappearance of the last rudiments. The different functional significance of homologous organs in different groups indicates that during phylogenetic development, the functions of these organs have undergone change (especially the flying limbs of a bird and a bat, the flipper of a whale, the digging limb of a mole, and the grasping limb of a human). The form of an organ and its function change, "mutually conditioning each other" (Engels), and perhaps the change of function (A. Dohrn) plays a leading role in the transformations of the organ. This change usually proceeds in such a way that one of the secondary functions of the organ becomes the main one. The phylogenetic direction in Comparative Anatomy developed with extraordinary speed. Its founder himself, Gegenbaur, occupied the most prominent place due to his scientific output. In his major works devoted to the skeleton of lower vertebrates, the foundations of the modern theory of the skull and the theory of limbs were laid. Among his students, one should mention M. Fürbringer (Max Fürbringer, 1846-1920) and H. Braus (Hermann Braus, 1868-1924), who faithfully followed his path. Although Gegenbaur recognized the importance of the embryological method in Comparative Anatomy, he treated it quite critically. Other researchers, however, used it widely and achieved great results in this direction. In England, F. Balfour (Francis Balfour, 1851-82) compiled the first major comparative-embryological comparison in the light of evolutionary doctrine. In Russia, A. O. Kovalevsky (1840-1901), V. V. Zalensky (1847), A. N. Severtsov, and others also followed mainly the embryological method. As Comparative Anatomy developed, the insufficiency of the methods became increasingly clear: comparison, even in conjunction with the embryological method, made it possible to construct various phylogenetic conclusions only with greater or lesser probability; therefore, modern Comparative Anatomy (especially in America) also makes wide use of paleontological data for a more solid substantiation of its conclusions (Osborn, Gregory, Goodrich, Watson, A. N. Severtsov, P. P. Sushkin). Other researchers followed the path of experiment, such as W. Roux (Wilhelm Roux, 1850-1924), H. Braus, and others. Along with this, the tasks of comparative anatomy are also expanding. Not content with merely establishing phylogenetic connections, it is moving on to the study of the regularities of the evolutionary process itself (A. N. Severtsov). The path of the researcher-anatomist studying the structure of animals is always a path of analysis. By dissecting a dead body into parts, organs, and tissues, he forgets that before him is a harmonious whole, which in its essence is indivisible. In Comparative Anatomy and phylogenetics, analysis is always followed by synthesis, but as a result of this synthesis, often only a lifeless scheme was obtained; therefore, recently, a reaction against the phylogenetic direction has grown, and a new path of studying the organism as a whole in connection with its surrounding biological environment has emerged. The structure of an organ is studied in connection with its function; the correlative connection between parts of the organism and the relationships between the anatomical construction of the latter and the conditions of the external environment are studied. The teaching of Comparative Anatomy in Germany was usually connected with the chair of human anatomy at medical faculties of universities; in other countries, the teaching of Comparative Anatomy is more often entrusted to zoologists. The significance of Comparative Anatomy for medicine boils down to illuminating the structure of the human organism from the point of view of its origin, which explains not only the existence of various kinds of rudiments (gill slits, appendix), but also a number of structures in general that are often the causes of diseases (for example, inguinal hernia), as well as a number of observed anomalies. In the USSR, comparative anatomical research is carried out at the Institute of Evolutionary Morphology of the Academy of Sciences of the USSR, at the Institute of Comparative Anatomy of the 1st Moscow State University, and in the zootomical cabinets of other universities. P. F. Lesgaft was the first in the pre-revolutionary period to place the teaching of anatomy from the perspective of comparative anatomy. The largest society where much space is always devoted to comparative anatomical works is the German society "Anatomische Gesellschaft," which holds annual congresses and publishes "Anatomischer Anzeiger" and the proceedings of the congresses "Verhandlungen der Anatomischen Gesellschaft." In second place is the "Medizinisch-Naturwissenschaftliche Gesellschaft zu Jena," which publishes "Jenaische Zeitschrift für Naturwissenschaft." Other journals in Germany: "Gegenbaurs morphologisches Jahrbuch"; "Zoologische Jahrbücher, Abteilung Anatomie u. Ontogenie"; "Zeitschrift für die gesammte Anatomie", I. Zeitschrift für Anatomie u. Entwickelungsgeschichte, II. Ergebnisse der Anatomie und Entwickelungsgeschichte; "Zeitschrift für wissenschaftliche Biologie", Abt. A. Zeitschrift für Morphologie und Oekologie; "Zeitschrift für wissenschaftliche Zoologie". In England: "Quarterly Journal of microscopical science". In America: "Anatomical Record", "Journal of Morphology and Physiology". In the USSR: congresses of zoologists, anatomists, and histologists, which also publish their own proceedings. Comparative anatomical material is also published in the "Zoological Journal," Moscow, and in the "Russian Archive of Anatomy, Histology and Embryology," Leningrad. Publications of scientific societies: "Bulletin of the Moscow Society of Naturalists," biology section, "Proceedings of the Leningrad Society of Naturalists," and others. Collections on Comparative Anatomy in the USSR are available: at the Zoological Museum of the Academy of Sciences, the Institute of Comparative Anatomy of the 1st Moscow State University, and in the zootomical cabinets of other universities, which also have corresponding libraries.