« PrécédentContinuer »
ANATOMY is a comprehensive term, which includes several closely related branches of study. Primarily it is employed to indicate the study of the parts which build up the body, and the relationship which these present to each other. But the structure of an individual is not the same at all stages of its life, for many changes occur during the period of its existence. The ovum and the spermatozoon, which are the starting-points of every individual, are very different from the finished organism as represented by the adult, and the series of changes through which the organism passes until its structure is perfected and full growth is attained constitute the phenomena of development. The general term “development” includes not only the various and striking structural changes which occur during the intra-uterine life of the individual, to the study of which the term embryology is more specially applied, but also many growth processes which occur after birth, such as the later stages in the ossification and growth of the bones, the eruption of the two series of teeth, the adjustment of the vascular system to its new requirements, etc. The actual observation of the processes by which the parts of the body are gradually formed, and of the structural arrangements by means of which a temporary connexion is established between the ovum and the mother, through which an interchange of nutritive and other matters between the two takes place, renders embryology one of the most interesting of all the departments of anatomy. The term ontogeny also is used to denote the development of the individual. There is, however, another form of development, slower, but just as certain in its processes, which affects not only the individual, but all the members of the animal group to which it belongs. The theory of descent or evolution leads us to believe that between man of the present day and his remote ancestors there is a wide structural gap, which, if the geological record were perfect, would be seen to be completely occupied by long-lost intermediate forms. In the process of evolution, therefore, structural changes have gradually taken place which have modified the entire race. These evolutionary phases constitute the ancestral history or phylogeny of the race. Ontogeny and phylogeny are intertwined in a remarkable manner, and present certain extraordinary relationships. In other words, the ancestral evolutionary development appears to be so stamped upon an individual that it repeats certain of the phylogenetic stages with more or less clearness during the process of its own individual development. Thus, at an early period in the embryology of man evanescent gill-pouches appear which are comparable with those of a fish, whilst a study of the development of his lreart shows that it passes through transitory structural conditions similar, in many respects, to the permanent
conditions of the heart in certain of the lower animals. It is in connexion with this that the phrase has arisen that every animal in its individual development or ontogeny climbs up its own genealogical tree—a saying which, taking it even in the broadest sense, is only partially true.
The broader conceptions of anatomy, which are obtained by taking a general survey of the structural aspects of the entire animal kingdom, constitute morphology. The morphologist investigates the laws of form and structure, and in his generalisations he gives attention to detail only in so far as this is necessary for the proper establishment of his views. The knowledge of anatomy which is required by the student of medicine is different. It is essentially one of detail, and often details important from the practical and utilitarian points of view have little or no morphological value. This want of balance in the interest attached to anatomical facts, according to the aspect from which they are examined, so far from being unfortunate, affords the teacher the means of making the study of anatomy at once fascinating and instructive. Almost every fact which is brought under the notice of the student can be accompanied by a morphological or a practical application. These possibilities of application lighten a study which, presented to the student of medicine in any other way, would be at once dry and tedious.
Certain terms employed in morphology require early and definite explanation. These are homology, serial homology, and homoplasy. The same organ repeated in two different animals is said to present a case of homology. But the morphological identity between the two organs must be proved beyond dispute before the homology between them can be allowed. In deciding the identity the great and essential test is that the two organs in question should have a similar developmental origin. Thus, the fore-limb of a quadruped is homologous with the upper limb of man; the puny collar-bone of a tiger, the fibrous thread which is the only representative of this bone in the horse, and the strongly marked clavicle of the ape or man, are all, strictly speaking, homologous with one another. Homologous organs in different animals usually occupy a similar position and possess a similar structure, but not invariably so. It is not uncommon for a muscle to wander somewhat from its original position, and many cases could be quoted in which parts have become completely transformed in structure, either from disuse or for the purpose of meeting some special demand in the animal economy. In the study of the muscles and ligaments instances of this will be brought under the notice of the reader.
Often organs which perform totally different functions are yet perfectly homologous. Thus the wing of a bat or the wing of a bird, both of which are subservient to flight, are homologous with the upper limb of man, the office of which is the different one of prehension. Identity or correspondence in the function performed by two organs in two different animals is not taken into consideration in deciding questions of homology. The gills of a fish and the lungs of a higher vertebrate perform very much the same physiological office, and yet they are not homologous. The term analogy is often used to express functional correspondence of this kind.
In the construction of vertebrates and certain other animal groups a series of similar parts are repeated along a longitudinal axis, one after the other. Thus the series of vertebræ which build up the backbone, the series of ribs which gird round each side of the chest, the series of intercostal muscles which fill up the intervals between the ribs, the series of nerves which arise from the brain and spinal medulla, are all examples of this. An animal exhibiting such a condition of parts is said to present the segmental type of organisation. In the early stages of development this segmentation is much more strongly marked, and is to be seen in parts which subsequently lose all trace of such a subdivision. The parts thus repeated are said to be serially homologous. But there are other instances of serial homology besides those which are manifestly produced by segmentation. The upper limb is serially homologous with the lower limb: each is composed of parts which, to a large extent, are repeated in the other, and the correct adjustment of this comparison between the several parts of the upper and lower limbs constitutes one of the most difficult and yet interesting problems of morphology.
Homoplasy is a term which has been introduced to express a form of correspondence between organs in different animals which cannot be included under the term homology. Two animal groups, which originally have sprung from the same stem-form, may independently develop a similar structural character which is altogether absent in the ancestor common to both. Thus the common ancestor of man and the carnivora in all probability possessed a smooth brain, and yet the human brain and the carnivore brain are both richly convoluted. Not only this, but certain anatomists seek to reconcile the convolutionary pattern of the one with the convolutionary pattern of the other. What correspondence there is does not, in every instance, constitute a case of homology, because there is not in every case a community of origin. Correspondence of this kind is included under the term “ homoplasy.” Another example is afforded by the heart of the mammal and that of the bird. In both of these groups the ventricular portion of the heart consists of a right and a left chamber, and yet the ventricular septum in the one is not homologous with the corresponding septum in the other, because the common ancestor from which both have sprung possessed a heart with a single ventricular cavity, and the double-chambered condition has been a subsequent and independent development in the two groups.
Systematic Anatomy.-The human body is composed of a combination of several systems of organs, and the several parts of each system not only present a certain similarity in structure, but also fulfil special functions. Thus there are
1. The skeletal system, composed of the bones and certain cartilaginous and membranous parts associated with them, the knowledge of which is known as osteology.
2. The articulatory system, which includes the joints or articulations, the knowledge of which is termed arthrology.
3. The muscular system, comprising the muscles, the knowledge of which constitutes myology.
4. The nervous system, in which are included the brain, the spinal medulla, the ganglia of the spinal and cerebral nerves, the sympathetic ganglia, and the various nerves proceeding from and entering these. The knowledge of these parts is expressed by the term neurology. In this system the organs of sense may also be included.
5. The blood vascular and lymphatic system, including the heart, blood vessels, the lymph vessels, and the lymph glánds. Angeiology is the term applied to the knowledge of this system.
6. The respiratory system, in which we place the lungs, windpipe, and larynx.
7. The digestive system, which consists of the alimentary canal and its associated glands, and parts such as the tongue, teeth, liver, pancreas, etc.
8. The urogenital system, composed of the urinary organs and the reproductive organs—the latter differing in the two sexes.
The term splanchnology denotes the knowledge of the organs included in the respiratory, digestive, and urogenital systems.
9. The integumentary system consists of the skin, nails, hair, etc. The knowledge of this system is termed dermatology.
The numerous organs which form the various systems are themselves built up of tissues, the ultimate elements of which can be studied only by the aid of the microscope. The knowledge of these elements and of the manner in which they are grouped together to form the various tissues of the body forms an important branch of anatomy, which is termed histology.
The structure of the human body may be studied in two different ways. The several parts may be considered with reference to their relative positions, either as they are met with in the course of an ordinary dissection, or as they are seen on the surface of a section through the body. This is the topographical method. the other hand, the several systems of organs may be treated separately and in sequence. This constitutes the systematic method, and it is the plan which is adhered to in this treatise.
Descriptive Terms.-Anatomy is a descriptive science founded on observation, and in order that precision and accuracy may be attained it is necessary that we should be provided with a series of well-defined descriptive terms. It must
be clearly understood that all descriptions are framed on the supposition that the body is in the erect position, with the arms by the side, and the hands held so that the palms look forwards and the thumbs laterally. An imaginary plane of section, passing longitudinally through the body so as to divide it accurately into a right and a left half, is called the median plane, Fig. 1 (M.P.). When the right and left halves of the body are studied it will be found that both are to a large extent formed of similar parts. The right and left limbs are alike; the right and left halves of the brain are the same; there are a right and a left kidney and a right and a left lung, and so on. So far the organs are said to be symmetrically arranged. But still a large amount of asymmetry may be observed. Thus, the chief bulk of the liver lies to the right side of the median plane, and the spleen is an organ which belongs wholly to the left half of the body. Indeed, it is well to state that perfect symmetry never does exist. There always will be, and always must be, a certain want of balance between symmetrically placed parts of the body. Thus the right upper limb is, as a rule, constructed upon a heavier and more massive plan than the left, and even in those organs where the symmetry appears most