Disease and Its Causes by William Thomas Councilman (early reader books .TXT) 📕

Inheritance As A Factor In Disease.—The Process Of Cell Multiplication.—The Sexual Cells Differ From The Other Cells Of The Body.—Infection Of The Ovum.—Intra-Uterine Infection.—The Placenta As A Barrier To Infection.—Variations And Mutations.—The Inheritance Of Susceptibility To Disease.—The Influence Of Alcoholism In The Parents On The Descendants.—The Heredity Of Nervous Diseases.—Transmission Of Disease By The Female Only.—Hemophilia.— The Inheritance Of Malformations.—The Causes Of Malformations.—Maternal Impressions Have No Influence.—Eugenics.

The question of inheritance of disease is closely associated with the study of infection, and the general subject of heredity in its bearing on disease can be considered here. By heredity is understood the transference of similar characteristics from one generation of organisms to another. The formation of the sexual cells is a much more complex process than that of the formation of single differentiated cells, for the properties of all the cells of the body are represented in the sexual cells, to the union of which the heredity transmission of the qualities of the parents is due. In the nucleus of all the cells in the body there is a material called chromatin, which in the process of cell division forms a convoluted thread; this afterwards divides into a number of loops called chromosomes, the number of which are constant for each animal species. In cell division these loops divide longitudinally, one-half of each going to the two new cells which result from the division; each new cell has one-half of all the chromatin contained in the old and also one-half of the cytoplasm or the cell material outside of the nucleus. The process of sexual fertilization consists in the union of the male and female sex cells and an equal blending of the chromatin contained in each (Fig. 22). In the process of formation of the sexual cells a diminution of the number of chromosomes contained in them takes place, but this is preceded by such an intimate intermingling of the chromatin that the sexual cells contain part of all the chromosomes of the undifferentiated cells from which they were formed. The new cell which is formed by the union of the male and female sexual cells and which constitutes a new organism, contains the number of chromosomes characteristic of the species and parts of all the chromatin of the undifferentiated cells of male and female ancestors. As a result of this the most complicated mechanism in nature, it is evident that in a strict sense there can be no heredity of a disease because heredity in the mammal is solely a matter of the chromosomes and these could not convey a parasite. The new organism can, however, quickly become diseased and, by the transference of disease to it and by either parent, there is the appearance of hereditary transmission of disease, though in reality it is not such. The ovum itself can become the site of infection; this, which was first discovered by Pasteur in the eggs of silkworms, takes place not infrequently in the infection of insects with protozoa. In Texas fever the ticks which transmit the disease, after filling with the infected blood, drop off and lay eggs which contain the parasites, and the disease is propagated by the young ticks in whom the parasites have multiplied. The same thing is true in regard to the African relapsing or tick fever, which is also transferred by a tick. In the white diarrhoea of chickens the eggs become infected before they are laid and the young chick is infected before it emerges from the shell. It is highly improbable, and there is no certain evidence for it, that the extremely small amount of material contributed by the male can become infected and bring infection to the new organism. In the cases in which disease of the male parent is transferred to the offspring, it is either by an infection of the female by the male, with transference of the infection from her to the developing organism, or with the male sexual cells there may be a transference to the female of the infectious material and the new organism may be directly infected. No other disease in man is so easily and directly transferred from either parent to offspring as is syphilis, and the disease is extremely malignant for the foetus, usually causing death before the normal period of intra-uterine development is reached.

Fig. 21.—Diagram Showing The Relation Of The Sexual Cells To The Somatic Cells Or Those Of The General Body. The sexual cells are represented to the left of the line at the bottom of diagram and are black. From the fertilized ovum at the top there is a continuous cell development, with differentiation represented in the cell groups of the bottom row. It is seen that the sexual cells are formed directly from the germ cell and contain no admixture from the cells of the body.

Fig. 21.—Diagram Showing The Relation Of The Sexual Cells To The Somatic Cells Or Those Of The General Body. The sexual cells are represented to the left of the line at the bottom of diagram and are black. From the fertilized ovum at the top there is a continuous cell development, with differentiation represented in the cell groups of the bottom row. It is seen that the sexual cells are formed directly from the germ cell and contain no admixture from the cells of the body.

The mother gives the protection of a narrow and unchanging environment and food to the new organism which develops within the uterus, and there is always a membranous separation between them. Disease of the mother may affect the foetus in a number of ways. In most cases the membrane of separation is an efficient guard preventing pathogenic organisms reaching the foetus from the mother. In certain cases, however, the guard can be passed. In smallpox, not infrequently, the disease extends from the mother to the foetus, and the child may die of the infection or be born at term with the scars resulting from the disease upon it. Syphilis in the mother in an active stage is practically always extended to the foetus. We have said that in an infectious disease substances of an injurious character are produced by bacteria, and such substances being in solution in the blood of the infected mother can pass through the membranous barrier and may destroy the foetus although the mother recovers from the infection.

Fig. 22.—Diagrammatic Representation Of The Process Of Fertilization. (Boveri.) In the first cell (a) the ovum is shown in process of fertilization by the entering spermatozoon or male sexual element. In the following cells there is shown the increase in amount of the male material and the final intimate commingling in g which precedes the first segmentation. g represents a new organism formed by the union of the male or female cell but differing from either of them.

Fig. 22.—Diagrammatic Representation Of The Process Of Fertilization. (Boveri.) In the first cell (a) the ovum is shown in process of fertilization by the entering spermatozoon or male sexual element. In the following cells there is shown the increase in amount of the male material and the final intimate commingling in g which precedes the first segmentation. g represents a new organism formed by the union of the male or female cell but differing from either of them.

Living matter is always individual, and this individuality is expressed in slight structural variations from the type of the species as shown in an average of measurements, and also in slight variations in function or the reactions which living tissue shows towards the conditions acting upon it. The anatomical variations are more striking because they can be demonstrated by weight and measure, but the functional variations are equally numerous. Thus, no two brains react in exactly the same way to the impressions received by the sense organs; there are differences in muscular action, differences in digestion; these variations in function are due to variations in the structure of living material which are too minute for our comparatively coarse methods of detection. In the enormous complexity of living matter it is impossible that there should not be minute differences in molecular arrangement and to this such functional variations may be due. Chemistry gives us a number of examples of variations in the reaction of substances which with the same composition differ in the molecular arrangement. Even in so simple a mechanism as a watch there are slight differences in structure which gives to each watch certain individual characteristics, but the type as an instrument constructed for recording time remains. In the fusion of the chromosomes of the male and female sexual cells, to which the hereditary transmission of the ancestral qualities to the new offspring is due, there are differences in the qualities of each, for the individuality of the parents is expressed in the germ cells, and the varying way in which these may fuse gives to the new cell qualities of its own in addition to qualities which come from each ancestor, and from remote ancestors through these. The qualities with which the new organism starts are those which it has received from its ancestors plus its individuality. The fact that the sexual cells are formed from the early formed cells of the new organism which represent all of the qualities of the fertilized ovum or primordial cell, renders it unlikely that the new offspring will contain qualities which the parents have acquired. The question of the inheritance of characteristics which the parents have acquired as the result of the action of environment upon them is one which is still actively investigated by the students of heredity, but the weight of evidence is opposed to this belief.

In the new organism the type of the species is preserved and the variations from the mean to which individuality is due are slight. We are accustomed to regard as variations somewhat greater departures from the species type than is represented in individuality, but there is no sharp dividing line between them.

Very much wider departures from the species type are known as mutations. Such variations and mutations, like individuality, may be expressed in qualities which can be weighed and measured, or in function, and all these can be inherited; certain of them known as dominant characteristics more readily than others, which are known as recessive. If these variations from the type are advantageous, they may be preserved and become the property of the species, and it is in this way that the characteristics of the different races have arisen. Certain of the variations are unfavorable to the race. The varying predisposition to infection which undoubtedly exists and may be inherited represents such a variation. Tuberculosis is an instance of this; for, while the cause of the disease is the tubercle bacillus, there is enormous difference in the resistance of the body to its action in different individuals. The disease is to a considerable extent one of families, but while this is true the degree of the influence exerted by heredity can be greatly overestimated. The disease is so common that