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The chromatic substance is aggregated to form first a fine and afterwards a thicker thread or spirem. At the same time, a spindle of achromatic fibrils appears between the two daughter centrosomes, and the nuclear membrane disappears.
As soon as the achromatic spindle is definitely established the chromatic thread breaks up into a number of segments, the chromosomes, which arrange themselves around the equator of the achromatic spindle.
The chromosomes may be V-shaped, rod-like, cuboidal or spheroidal, and each may be a single structure, or it may consist of two or four parts which are closely bound together. There is evidence which tends to support the belief that, whether the chromosome appears to consist of one, two, or more segments, its constituent particles are derived partly from the maternal and partly from the paternal ancestor of the cell; and it is believed that the maternal and paternal portions undergo similar division during the last three phases of mitosis. In any case, whether the chromosomes are single or compound structures, each becomes
FIG. 6.--SCHEMA OF ANIMAL CELL IN META- attached to, or very closely associated with, one
PHASE OF HOMOTYPE MITOSIS.
of the fibrils of the achromatic spindle.
pole of spindle Achromatic spindle
At the end of the prophase the nucleus as such, and the nucleolus, have entirely disappeared, and the cell body contains, in their place, two centrosomes, an achromatic spindle, and the chromosomes. The centrosomes lie at the opposite poles of the achromatic spindle with the granules of the protoplasm grouped radially around them, and the chromosomes are grouped round the equator of the achromatic spindle.
FIG. 7.-SCHEMA OF ANIMAL CELL AT END the rods or loops dividing longitudinally; and
OF ANAPHASE OF HOMOTYPE MITOSIS.
the division, in all cases, commences at the point where the chromosome is in relation with the fibrils of the achromatic spindle.
The Metaphase.-During the metaphase each chromosome divides into two equal parts,
The Anaphase.-In the anaphase the halves of the chromosomes, i.e. daughter chromosomes, move towards the opposite poles of the achromatic spindle, and when they reach the vicinity of the daughter centrosomes the anaphase ends and the telophase begins.
The Telophase.-At the end of the anaphase, or the commencement of the telophase, a constriction appears around the periphery of the cell, at the level of the equator of the achromatic spindle. After its appearance the constriction gradually deepens until the cell is
FIG. 8.-SCHEMA OF ANIMAL CELL AT END OF
TELOPHASE OF HOMOTYPE MITOSIS. The cell has divided into two daughter cells. Red and blue indicate the original paternal completely divided into two halves, the daughter cells, each of which contains the
and maternal derivatives.
typical number of chromosomes, and a portion of the achromatic spindle.
The Resting Stage. During the resting stage, which lasts for a variable period, a nucleus is formed in each daughter cell by the appearance of a nuclear membrane around the chromosomes, as they repass first to the thread-like and then to the granular form of chromatic substance, and by the reappearance of a nucleolus. The cell increases in size also.
The Period of Cell Life.-The period of cell life varies, but in all cases it ltimately ends in death; for a time comes when cells no longer transmit to their
descendants the power of division, or the capability of growth and function. If it were not so, growth and function, or at least maintenance and function, would continue uninterruptedly, and in the absence of accident or disease individual life would continue for ever, and " old age" would be unknown.
It appears, therefore, that the ancestors of certain tissue cells are capable of producing only a certain number of descendants, which grow to the normal size and perform their proper functions for a more or less fixed period, whilst in other cases the power of division appears to be transmitted continuously, but the more remote descendants become less and less capable of performing their proper functions. The result in both cases is the same; gradual decay, terminating in death.
Heterotype Mitosis. In ordinary or homotype mitosis the chromosomes are divided into equal parts, and, when the process of cell division is completed, each daughter cell possesses the same number and same kind of chromosomes as the mother cell from which it was derived (Figs. 3-8). In heterotype mitosis, the number of chromosomes is reduced during the cell division, and each daughter cell possesses only half the number of chromosomes that was present in the mother cell.
The details of the division of the chromosomes during heterotype mitosis differ in different groups of animals, but the end is the same in all cells in which the process occurs, and is the reduction of the number of the chromosomes in the daughter cells to half the number typical for the ordinary cells of the animal.
The most typical form of heterotype mitosis is seen during the first maturation division of many germ cells, in which, during the spirem or thread-like stage of the chromatic substance, careful examination of the thread shows that it consists of a number of alternate segments attached end to end, the number of segments corresponding with the number of the chromosomes typical for the ordinary cells of the animal. Towards the end of the prophase, the segments of the thread become attached together in pairs which form a number of twin chromosomes. These arrange themselves around the equator of the achromatic spindle, and it is obvious that the number of twin chromosomes is only half the number of the chromosomes originally present in the cell (Figs. 12 and 13). (See note 1, p. 79.)
The total number of chromatic segments is still the same, for each twin chromosome consists of two ordinary chromosomes attached side to side.
The process of reduction takes place during the metaphase, when the two segments of each twin chromosome become separated from one another. During the anaphase the separated segments pass to the opposite poles of the achromatic spindle, and when the telophase is completed the number of chromosomes in each daughter cell is half that which was present in the mother cell (Figs. 12-19).
In some cases, at the commencement of heterotype mitosis, the chromosomes are not arranged in pairs as twins, but in groups of four, called tetrads, each tetrad consisting of a pair of dyads. In those cases the two dyads of each tetrad are separated from one another during the metaphase, and when the telophase is completed each daughter cell possesses only half the number of chromatic particles which were present in the mother cell.
It is known that a cell which contains only half the typical number of chromosomes can divide once, therefore from each original cell which underwent heterotype mitosis four grand-daughter cells may be produced. It is still uncertain, however, whether or not cells which contain only half the typical number of chromosomes can further subdivide, or whether they can continue to live and function. So far as the observations made can be relied upon, it appears that such cells either die or they unite with another cell containing half the typical number of chromosomes to produce a new cell which contains the typical number of chromosomes and which possesses also the capability of reproducing itself by division.
The Gametes.-The gametes are the germ elements by whose union, in pairs, new individuals are produced.
They are of two kinds, female gametes or ova and male gametes or spermatozoa. Both female and male gametes are modified cells, by means of which hereditary characteristics are transmitted from generation to generation, and they are derived from cells called primitive germ cells, whose origin will be considered in association with the development of the germinal layers.
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 glands. 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. On 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