An atomism

(Biol.) Anatomical structure regarded as the basis of life and its phenomena; the doctrine that vitality is to be accounted for by anatomical structure; the application, exhibition, or portrayal of anatomical features, as in works of art. Anatomy, Comparative. The study of the structure of animals, including man, and the comparison of the various organs with one another. In this respect it differs from Descriptive Anatomy , which is the investigation and description of organs without reference to their relations in different animals; while Physiology treats of the functions or uses of organs. The term was formerly used to denote the study of the structure of the lower animals in contrast to the study of the structure of man, but in this sense it is now practically obsolete. Various terms are given to those branches of anatomy which treat of particular cla es of organs. Thus, the study of minute structures, or parts of organs, which nece itates the use of the microscope, is called Histology ; and, though this is sometimes spoken of as a separate branch of research, it cannot properly be so considered. The study of the nervous system constitutes Neurology ; of the circulatory system, Angiology ; of the viscera, Splanchnology; of the joints, Syndesmology ; of the muscles, Myology ; of the teeth, Odontology ; and of the bones, Osteology .-Comparative Anatomy in its widest sense may be considered as of very modern origin; for, while Aristotle is generally regarded as the founder of this branch of science, from his day until the time of Cuvier it progre ed but slowly, although for practical reasons much was done that the more they differ in structure, the more widely should they be separated.-Formerly, animals were' cla ified by their external appearance, or even by their habits; and this resulted in whales being grouped with fishes, and some lizards with snakes-mistakes which even a slight knowledge of anatomy would have prevented, since investigation shows that in every part of their anatomy whales and fishes are IMG:content-0458.png:[graphic] mals, and that without a knowledge of their structure they cannot be properly cla ified. He was the founder of the science of paleontology; and while others had somewhat hesitatingly recognized that fo il bones were the remains of extinct quadrupeds, it remained for Cuvier to demonstrate that their forms and affinities could be deduced from a study of the anatomy of existing species. This he did by his "law of the correlation of growth," the principle that certain modifications of structure will be found a ociated with others; that, for example, the teeth of a ruminant would be found in company with hoofs and not with claws, and that conversely the teeth of a carnivore and the limbs of a ruminant would not be found in the same animal. In the light of our more complete knowledge of fo il forms, we know that Cuvier trusted too implicitly to this principle, which loses somewhat of its force as we go backwards in time; but with the mammals of the Paris Basin, with which Cuvier largely dealt, the "law" holds good. There is even to-day much popular misconception as to the po ibilities of restoring animals from teeth or fragments of the skeleton, or of reconstructing a fish from a scale; but while in certain cases much may be done with a single tooth, in others the task may be difficult, or practically impo ible, even when a goodly portion of the skeleton is available. Comparative anatomy often touches physiology closely, especially in studying the lower or simpler forms of life, where the use of an organ may determine its homologies, or identity with organs of other animals, as well as its relation to other parts. On the other hand, an acquaintance with the functions and relations of a given part in one animal may enable the physiologist to judge of its importance in another, or even in man, and to determine on the po ibility of operating upon or removing it in case of disease. Two other branches of science are so closely allied to Comparative Anatomy that at times a knowledge of one is needed for a proper interpretation of the facts of the other. These are Embryology, the study of the development of animals from the egg onwards, and Palæontology, the study of extinct animals. Our entire knowledge of xtinct vertebrates practically depends on the study of their skeleton and teeth, particularly the latter, since the teeth, being the hardest parts of the body, are often preserved when all other portions have disappeared. And, on the other hand, an acquaintance with extinct forms is needed for the correct understanding of the meaning of many structures found in existing species; therefore, palæontology may almost be considered as a branch of comparative anatomy. It will indeed be found that authorities are not agreed as to the exact limits to be set on the scope of anatomy, as was noted when speaking of histology; but this is not surprising when it is considered how intimate are the relations existing between it and other divisions of science. The relation of Comparative Anatomy to the theory of evolution is very obvious and direct; for, if existing animals have descended from common ancestral forms, we ought to find them connected by a structural thread, as it were, and be able to trace certain resemblances in their various parts. We should also meet with differences due to departures from the original form, and many of these should be clearly due to adaptations of the same organs to different uses. Finally, by the aid of fo ils, it should be po ible to trace the relationship of animals which now seem widely separated through the intermediate forms having become extinct. We should also expect, if the theory of evolution be true, that as we went backward in time we should find that the differences between animals were le sharply marked than at the present day; that, as the zoologist puts it, they were more "generalized," or built more according to one plan. And this is exactly what we do find, for the palæontologist, and the paleontologist is merely a comparative anatomist who deals with extinct animals, has discovered many fo il forms in which are combined characters which are not found a ociated in living animals. One of the most notable triumphs of the anatomist was the finding of Phenaco dus in the Eocene deposits of the western United States. This mammal has very diverse relationships, having on the one hand affinities with the hoofed quadrupeds and on the other, though distantly, relations with the carnivores and lemurs. The discovery of the animal was predicted by Cope, Marsh, and Kowalesky, from the fact just mentioned, viz., that specialized forms have been preceeded by those of a more generalized structure; so that, with a knowledge of existing animals and some of their predece ors, it was po ible to go a step farther and say what in turn should have preceeded these. -The bearing of Comparative Anatomy on the cla ification of animals is quite as plain as its bearing on the question of evolution. The object of zoological cla ification is to arrange animals as nearly as po ible in a natural manner, and to expre their relations to one another; it is simply a sorting over of animals and placing together those which are alike, just as in arranging a library we would keep together books that treated of similar subjects. It is clear that the more closely two animals resemble one another in structure, the greater are the chances of their being related; and structure the whale resembles the other mammals, that great group of vertebrates to which man himself belongs. To-day, cla ification rests on a solid foundation of comparative anatomy; and animals, like books, are sorted according to their contents and not according to their bindings, being divided into groups distinguished by the po e ion of some structural feature in common. We are thus enabled not only to cla ify or arrange existing animals, but those which long ago became extinct. External characters are still employed, but mainly in the determination of the smaller divisions, and especially species, which are the ultimate divisions or units of cla ification.-By combining our knowledge of the anatomy of existing and extinct animals and noting the changes that have taken place in the structure of the various groups as they have succeeded one another in time, we have an idea of their lines of descent, or phylogeny. Uniting comparative anatomy, palæontology and embryology, we obtain our fullest knowledge of the structural relations of animals to one another; and this constitutes morphology (the science of structure), in distinction to physiology (the science of the use of structure). Since morphology rests on a foundation of anatomy, the difficulty or drawing any sharp line of demarcation between them can be readily seen. But, in a general way, it may be said that it is the province of anatomy to furnish facts and the province of morphology to interpret them. The one traces the modifications of a given organ, or series of organs; the other explains their bearings on the relationships of animals. In order to better illustrate the scope and methods of Comparative Anatomy, it will be well to examine and compare the variations of some organ, or system of organs, as shown by the great divisions of the animal kingdom. The digestive apparatus is a good one for this purpose, not only because its modifications can be readily followed and their differences readily appreciated, but because the function of nutrition is perhaps the most important of all. A creature may exist without any obvious nervous or circulatory system, but it must eat in order to live. Of nece ity this will be, indeed, but the briefest glance at the subject, since to describe the digestive apparatus as it is found even in the larger groups of animals, noting the variations in structure, the apparent exceptions to the general plan and the simplification of parts brought about through degeneration, would be to usurp the functions of a text book. The primary divisions, or branches, of the animal kingdom are termed the phyla , and are based on the common po e ion of some fundamental anatomical feature. Authorities are not agreed as to the exact number of even these primary divisions, but that most generally accepted is eight, although as our knowledge of extinct forms and of the embryonic changes undergone by various animals increases, it is entirely probable that this number may also be increased. It is quite probable, too, that there may be more or le shifting about of forms, and that animals now placed in one category may be transferred to another. Formerly the animal kingdom was divided into the two groups of Invertebrates and Vertebrates, but anatomy long ago showed that this arrangement was by no means as natural as it seemed; that some of the so-called Invertebrates were really Vertebrates, and that several distinct plans of structure prevailed among the others. The eight phyla are as follows: I. PROTOZOA.-Minute (often microscopic) animals, consisting of a single cell, destitute of blood, nerves and other organs differentiated for the various functions of life. They include the Foraminifera, Infusoria and similar low animals. II. PORIFER A.-Fixed, aquatic, compound animals, composed of numerous individuals disposed about a common cavity through which water flows in and out. The body wall is often composed of two layers only ectoderm and endoderm and there are no tentacles. Reproduction may take place, as in the Protozoa, by division; or young may be developed from eggs. This division comprises the Sponges. III. CEL ENTER AT A.-Animals composed of numerous cells, usually arranged in two layers only; body cavity formed entirely by the digestive tube, which has but a single opening; no circulatory nor excretory system. Tentacles, for grasping food, are present, and in some species a rudimentary peripheral nervous system. To this group belong the jelly-fishes, sea anemones, and corals. IV. VERMES--Body usually elongate, often segmented; digestive system distinct, sometimes forming the sole body cavity, sometimes suspended in a separate cavity; circulatory system imperfect or wanting; nervous system, a ring about the mouth and acce ory ganglia. This branch contains the worms, in the widest sense of the word. V. ECHINODERM AT A.-Digestive system distinct from the general body cavity; circulatory system imperfect, largely distinct; nervous system, a ring about the mouth with radiating branches; integument entirely or partly calcified. This division contains the starfishes, sea-urchins, and sea-cucumbers. VI. ARTHROPOD A.- Digestive system complete and distinct; circulatory system, a central contractile organ, with branches open at the ends; nervous system, a ring about the gullet, and usually a line of ganglia along the under side of the body; body segmented and provided with jointed legs or appendages. Crustacea, insects, spiders, . VII. MOLLUSCA.-Digestive apparatus complete and distinct; circulatory system incomplete through the opening of the ends of the branches; nervous system, a ring about the gullet, with acce ory ganglia and connected with another ganglion on the lower side; body not segmented, and usually covered with a hard, calcareous shell. The shell-fish, in the broad acceptation of the word. VIII. VERTEBRATA.-Digestive and respiratory apparatus. distinct and complete; circulatory system forming a complete circuit; nervous system lying along the dorsal side of the body (spinal cord) and usually having an enlargement (brain) at the anterior end; an internal cartilaginous or bony skeleton present. The vertebrates, or back-boned animals, including some very simple primitive and degenerate forms, recognizable as vertebrates only through careful study of their anatomy and development. Starting with those simple animals which consist of but a single ceil, it is found that in many food may be taken in and the digested material rejected at any part of the body; neither is there any distinct digestive apparatus, food being a imilated freely by the contents of the cell. But even in some of these low forms-the Infusoria, for example-we find the starting point of an alimentary canal, since food is taken in at one definite point and the waste products of digestion rejected at another. A step above these, in such animals as the sea anemones, there is a pocket-like cavity in which digestion takes place, material which is not a imilated being cast out by the way it entered. The walls of this primitive stomach also serve for purposes of respiration, and there may be a suggestion of the more complicated organs of higher animals in the shape of finger-like projections which serve to convey nutriment into various parts of the body. Among worms we, for the first time, meet with an alimentary canal in the form of a tube, often much convoluted, opening at either end of the body and often having a certain amount of division into three portions; a hint of the gullet and stomach; and the small and large intestine of vertebrates. Hard, tooth-like bodies are also frequently present in the mouth; while in this group, too, we meet with additional glandular structures, which may be regarded as the forerunners of such organs as the salivary glands and liver of still more specialized forms. By no means all worms have so good an alimentary canal as this, for some species have merely a tube with various branches penetrating the soft substance of the body, the same opening serving for ingestion and excretion; while in some degenerate parasitic forms there is no canal whatever, food being directly absorbed through the surface of the body. Although the alimentary canal of the Echinoderm at a-the star-fishes, sea-urchins, and sea cucumbers-is in itself quite simple, it shows a decided advance of structure, in the fact that it is not a mere cavity in the general substance of the body, but an independent tube suspended in a body cavity. As in previous instances, part of the digestive tube serves for purposes of respiration, but in the sea-cucumbers there are certain curious structures attached to its hinder portion which seem to be also connected with locomotion. These structures are hollow, branching, treelike organs, which can at will be filled with water, and when this is done, it renders the back part of the body somewhat rigid, thus affording a point of support from which the body can be pushed forward. So in these creatures the alimentary canal performs the three functions of digestion, respiration, and locomotion. Arthropods there is a still farther advance in the digestive system, for not only are the functions of the alimentary canal strictly confined to digestion, but parts of it are so differentiated as to perforni special duties; thus, the interior portion is mainly concerned in swallowing and preparing food, the middle portion doing the main work of a imilation, the hinder portion being charged with getting rid of waste products. In this group a crop, or dilatation of the front part of the digestive tube, is often present, serving to retain the food in order that there may be more time to prepare it for digestion. Many arthropods, and particularly insects, have appendages which serve as salivary glands, while a liver is quite generally represented; organs regarded as connected with the products of excretion being also present. There is much difference in the length and complexity of the digestive tube among the members of this group, according to the character of their food, and the direct relation between the two is well shown by the fact that when the caterpillar transforms to a butterfly great changes take place in the organs of digestion in order that they may be adapted to the new conditions of life. Among mollusks the digestive tube is quite distinct from the general cavity of the body, and, as it is always longer, forms several coils or loops. It is also more or le clearly divided into three separate regions, and there is frequently an en large ment of the gullet to form a crop in which food is temporarily retained before pa ing to the stomach proper. In many species the mouth is provided with an elaborate arrangement of horny teeth, while the Cephalopods (squids, .), have a beak suggestive of that of a parrot, Sometimes the stomach is strong and muscular, acting as a gizzard to grind food, and sometimes it is divided into two or more por tions, one of which acts as a gizzard, the others treat In ing the food chemically by the admixture of secretions! similar to the gastric juice. The salivary glands and liver are well developed, and on the whole the digestive system shows a great advance in complexity. Vertebrates vary greatly in the development of the digestive system, and there is a wide difference between the simple tube of the lancelet ( Branch ios tom a ) and the complex apparatus of a mammal; but in all the following divisions of the alimentary canal may distinguished, viz.: month, gullet, stomach, small and large intestine. There may also be present salivary, mucous, thyroid and thymus glands, liver and pancreas, while the surface of the digestive tract may be further extended by folds and its effectivene increased by numerous small glands in the stomach and intestine, any one of which is a more complicated structure than one of the simple animals with which we started. There is indeed a vast distance between the Ameba and Man, and it is the province of Comparative Anatomy to furnish the facts with which to bridge over the gap, to show how the simplicity of the one may be connected with the complexity of the other, and to trace the various steps by which such great structural differences are brought about. From the comparative study of the digestive apparatus we learn that there is no constant and regular advance in pa ing from group to group, but that in some species of a division the system may be le developed than in some members of the group immediately lower in the scale. In other words, there is some "overlapping." Neverthele , on the whole, progre is from simplicity to complexity; so that starting with such a form as Ameba, in which food enters at any part of the body and is digested at any point within, we finally reach the complex digestive system of mammals, where one portion of the alimentary canal is devoted to preparing food for digestion, another to the extraction of the nutritious portion, and still a third to removing waste products. So, too, we find that while in the simpler animals respiration is more or le performed by the digestive apparatus, and that it may even aid in locomotion, in its more perfected state it is devoted to digestion only. Not only this, but as we come upwards we find organs, such as the salivary glands and liver, added to the digestive tube in order that it may the better perform its work. These facts not only show that the same organ may differ somewhat in its function in different animals, but also that the progre ion is from generalization to specialization; that while in the simpler animals the same part may serve for varied uses, in the higher forms the various functions of life are performed by special organs or systems of organs. And this is why the zoologist speaks of animals as being "generalized" or "specialized" in structure; although it must be remembered that, while an animal may be generalized as regards its entire structure, it may be highly specialized in some way in order to adapt it to a certain mode of life. It will also be found, although little stre has been laid on the fact in this brief review, that there is a direct relation between the nature of the food and the character of the digestive apparatus, and that differences between the organs of different species of animals may be due to adaptations for some particular use and not to any real fundamental difference in plan of structure.. With all these problems of structure, modification and function, anatomy is more or le intimately connected; and, while their final solution may rest with some other branch of science, the work must rest upon a foundation of Comparative Anatomy. Works of reference: Le ons in Elementary Anatomy , St. George Mivart (London, 1873); Anatomy of Vertebrated Animals , T. H. Huxley (New York, 1878); Comparative Anatomy and Physiology of Vertebrates, Richard Owen (London, 1866-1868, three vols.); Elements of Comparative Anatomy , Carl Gegenbaur (London, 1878); Comparative Anatomy of Verto brates, Robert Wiedersheim (London, 1886); Text Book of Comparative Anatomy , Arunold Long (London, 18911896, two vols. Does not in code vertebrates).