Disease and Its Causes

by W. T. Councilman

Fig. 13—Nerve Cells Of An English Sparrow (a) Cells after a day's full activity, (b) cells after a night's repose In (a) the cells and nuclei are shrunken and the smaller clear spaces in the cells are smaller and less evident than in (b). (Hodge)

The formative activity of cells is also essential to the normal state. Destruction of cells is constantly taking place in the body, and more rapidly in certain tissues than in others. Dried and dead cells are constantly and in great numbers thrown off from the surface of the skin: such epidermic appendages as the hair and nails grow and are removed, millions of cells are represented in the beard which is daily removed. Cells are constantly being destroyed on the intestinal surface and in the glands. There is an enormous destruction of the blood cells constantly taking place, certain essential pigments, as that of the bile, being formed from the hæmoglobin which the red blood corpuscles contain and which becomes available on their destruction. All such loss of cells must be made good by the formation of new ones and, as in the case of the nutritive and functional activity, the loss and renewal must balance. The formative activity of cells is of great importance, for it is by means of this that wounds heal and diseases are recovered from. This constant destruction and renewal of the body is well known, and it is no doubt this which has given rise to the belief, widely held, that the body renews itself in seven years and that the changes impressed upon it by vaccination endure for this period only. The truth is that the destruction and renewal of most tissues in the body takes place in a much shorter interval, and, as we shall see, this has nothing to do with the changes concerned in vaccination. All these activities of the cells vary in different individuals, in different parts and at different ages.

The lesions or injuries of the body which form so prominent a part of disease vary in kind, degree and situation, depending upon the character of the injurious agent, the duration of its action and the character of the tissue affected. The most obvious injuries are those produced by violence. By a cut, blood vessels are severed, the relations of tissues disturbed, and at the gaping edges of the wound the tissue usually protected by the skin is exposed to the air, resulting in destruction of the cells contained in a thin layer of the surface. The discoloration and swelling of the skin following a blow is due to rupture of vessels and escape of blood and fluid, and further injury may result from the interruption of the circulation.

By the application of heat the tissue may be charred and the albumen of the blood and tissue fluids coagulated. Living cells are very susceptible to the action of heat, a temperature of 130 degrees being the thermal death point, and even lower temperatures are fatal when their action is prolonged. The action of the heat may produce definite coagulation of the fluid within the cells in the same way that the white of an egg is coagulated. Certain of the albumens of the body coagulate at a much lower temperature than the white of the egg (as the myosin, one of the albumens of the muscle which coagulates at 115° F., egg white coagulating at 158° F.), and in addition to such coagulation or without it the ferments within the cell and to the action of which cellular activity is due may be destroyed.

In diseases due to parasites, the parasite produces a change in the tissue in its immediate vicinity often so great as to result in the death of the cells. The most general direct cause of lesions is toxic or poisonous substances, either introduced from without or formed in the body. In the case of the parasitic diseases the mere presence of the parasite in the body produces little or no harm, the injury being caused by poisons which it produces, and which act both locally in the vicinity of the parasite and at a distance, being absorbed and entering the blood stream. How certain of the poisonous substances act is easy to see. Strong caustics act by coagulating the albumen, or by the withdrawal of water from the cell. Other poisons act by forming stable chemical compounds with certain of the cell constituents and thereby preventing the usual chemical processes from taking place. Death from the inhalation of illuminating gas is due to the carbon monoxide contained in this, forming a firm chemical union with the hæmoglobin of the red corpuscles so that the function of these as oxygen carriers is stopped.

In order that most poisons may act, it is essential that they enter into the cell, and they cannot do this unless they are able to combine chemically with certain of the cell constituents. To this is due the selective action of many poisons. Morphine, for example, acts chiefly on the cells of the brain; strychnine acts on the cells of the spinal cord which excite motion and thus causes the characteristic muscular spasm. The poisonous substances produced by bacteria, as in the case of diphtheria, act on certain of the organs only. Different animal species owe their immunity to certain poisons to their cells being so constituted that a poison cannot gain entrance into them; pigeons, for example, cannot be poisoned by morphia. Individual variations play an important part also; thus, shellfish are poisonous for certain individuals and not so for others. Owing to the variability of living structures a substance may be poisonous at one time and not at another, as the following example shows. A man, very fond of crab meat, was once violently poisoned after eating crabs, being at that time seemingly in his usual state of health, and no illness resulted in others who had partaken of the same crabs. Two months later a hearty meal of crabs produced no ill result. There are also individuals so constituted that so simple a food as the egg is for them an active poison.

The lesions produced by the action of injurious conditions are usually so distinctive in situation and character that by the examination of the body after death the cause of death can be ascertained. The lesions of diseases may be very obvious to the naked eye, or in other cases only the most careful microscopic examination can detect even the presence of alterations. In the case of poisons the capacity of the cell for adaptation to unusual conditions is of great importance. It is probable that certain changes take place within the cells, owing to which the function can be continued in spite of the unusual conditions which the presence of the poison brings about. It is in this way that the habitual use of such poisons as morphine, alcohol and tobacco, to speak only of those best known, is tolerated. The cell life can become so accustomed to the presence of poisons that the cell activities may suffer in their absence.

Repair of the injuries which the body receives is effected in a variety of ways. We do not know how intracellular repair takes place, but most probably the cells get rid of the injured areas either by ejecting them, or chemical changes are produced in the altered cell substance breaking up and recombining the molecules. When single cells are destroyed, the loss is made good by new formation of cells, the cell loss stimulating the formative activity of the cells in the vicinity. The body maintains a cell and tissue equilibrium, and a loss is in most cases repaired. The blood fluid lost in a hæmorrhage is quickly restored by a withdrawal of the fluid from the tissues into the blood, but the cells lost are restored by new formation of cells in the blood-forming organs. The blood cells are all formed in bone marrow and in the lymph nodes, and not from the cells which circulate in the blood, and the stimulus to new cell formation which the loss of blood brings about affects this remote tissue.

In general, repair takes place most easily in tissues of a simple character, and where there is the least differentiation of cell structure for the purposes of function. A high degree of function in which the cell produces material of a complex character necessitates a complex chemical apparatus to carry this out, and a complicated mechanism is formed less easily than a simple one. In certain tissues the cells have become so highly differentiated that all formative activity is lost. Such is the case in the nerve cells of the brain and spinal cord, a loss in which tissue is never repaired by the formation of new cells; and in the muscles the same is true. The least differentiation is seen in those cells which serve the purpose of mechanical protection only, as the cells of the skin, and in these the formative activity is very great. Not only must the usual loss be supplied, but we are all conscious of slight injuries of the surface which are quickly repaired.