Definition of Disease : Part 4

Disease and its Causes
by William Thomas Councilman, M.D.

(Page 4 of 16)

Between these various surfaces is the real interior of the body, in which there are many sorts of living tissues, each, of which, in addition to maintaining itself, has some function necessary for the maintenance of the body as a whole. Many of these tissues have for their main purpose the adjustment and coördination of the activities of the different organs to the needs of the organism as a whole. The activity of certain of the organs is essential for the maintenance of life; without others life can exist for a time only; and others, such as the genital glands, while essential for the preservation of the life of the species, are not essential for the individual. There is a large amount of reciprocity among the tissues; in the case of paired organs the loss of one can be made good by increased activity of the remaining, and certain of the organs are so nearly alike in function that a loss can be compensated for by an increase or modification of the function of a nearly related organ. The various internal parts are connected by means of a close meshwork of interlacing fibrils, the connective tissue, support and strength being given by the various bones. Everywhere enclosing all living cells and penetrating into the densest of the tissues there is fluid. We may even consider the body between the surfaces as a bag filled with fluid into which the various cells and structures are packed.

The nervous system represents one of the most important of the enclosed organs. It serves an important function, not only in regulating and coördinating all functions, but by means of the special senses which are a part of it, the relations of the organism as a whole with the environment are adjusted. It consists of a large central mass, the brain and spinal cord, which is formed in the embryo by an infolding of the external surface, much in the same way that a gland is formed; but the connection with the surface is lost in further development and it becomes completely enclosed. Connected with the central nervous mass, forming really a part of it and developing from it, are the nerves, which appear as white fibrous cords and after dividing and subdividing, are as extremely fine microscopic filaments distributed to all parts of the body. By means of the nerves all impressions are conveyed to the brain and spinal cord; all impulses from this, whether conscious or unconscious, are conveyed to the muscles and other parts. The brain is the sole organ of psychical life; by means of its activity the impressions of the external world conveyed to it through the sense organs are converted into consciousness. Whatever consciousness is, and on this much has been written, it proceeds from or is associated with the activity of the brain cells just as truly as the secretion of gastric juice is due to the activity of the cells of the stomach. The activity of the nervous system is essential for extra-uterine life; life ceases by the cessation of circulation and respiration when either the whole or certain small areas of its tissue are destroyed. In intra-uterine life, with the narrow and unchanging environment of the fluid within the uterine cavity which encloses the foetus, life is compatible with the absence or rudimentary development of the nervous system. The foetus in this condition may be otherwise well developed, and it would be not a misuse of words to say that it was healthy, since it is adjusted to and in harmony with its narrow environment, but it would not be normal. The intra-uterine life of the unborn child, it must be remembered, is carried out by the transmission of energy from the mother to the foetus by means of the close relation between the maternal and foetal circulation. It is only when the free existence demands activities not necessary in intra-uterine life that existence without a central nervous system becomes impossible.

It is essential in so complicated a structure as the body that some apparatus should exist to provide for the interchange of material. The innumerable cell units of the body must have material to provide energy, and useless material which results from their activity must be removed. A household might be almost as much embarrassed by the accumulation of garbage and ashes as by the absence of food and coal. The food, which is taken into the alimentary canal and converted by the digestive fluids into material more directly adapted to the uses of cells, must be conveyed to them. A supply of oxygen is essential for the life of the cells, and the supply which is given by respiration must be carried from the lungs to every cell of the body. All this is effected by the circulation of the blood, which takes place in the system of branching closed tubes in which the blood remains. Certain of these tubes, the arteries, have strong and elastic walls and serve to convey and distribute the blood to the different organs and tissues. From the ultimate branches of the arteries the blood passes into a close network of tubes, the capillaries, which in enormous numbers are distributed in the tissues and have walls so thin that they allow fluid and gaseous interchange between their contents and the fluid around them to take place. The blood from the capillaries is then collected into a series of tubes, the veins, by which it is returned to the heart. This circulation is maintained by means of a pumping organ or heart, which receives the blood from the veins and by the contraction of its powerful walls forces this into the arteries, the direction of flow being determined as in a pump, by a system of valves. The waste products of cell life pass from the cells into the fluid about them, and are in part directly returned into the blood, but for the greater part pass into it indirectly through another set of vessels, the lymphatics. These are thin-walled tubes which originate in the tissues, and in which there is a constant flow towards the heart, maintained by the constant but varying pressure of the tissue around them, the direction of flow being maintained by numerous valves. The colorless fluid within these vessels is termed "lymph." At intervals along these tubes are small structures termed the lymph nodes, which essentially are filters, and strain out from the fluid substances which might work great injury if they passed into the blood. Between the capillary vessels and the lymphatics is the tissue fluid, in which all the exchange takes place. It is constantly added to by the blood, and returns fluid to the blood and lymph; it gives material to the cells and receives material from them.

In addition to the strength and elasticity of the wall of the arteries, which enables them to resist the pressure of the blood, they have the power of varying their calibre by the contraction or expansion of their muscular walls. Many of the organs of the body function discontinuously, periods of activity alternating with comparative repose; during the period of activity a greater blood supply is demanded, and is furnished by relaxation of the muscle fibres which allows the calibre to increase, and with this the blood flow becomes greater in amount. Each part of the body regulates its supply of blood, the regulation being effected by means of nerves which control the tension of the muscle fibres. The circulation may be compared with an irrigation system in which the water supply of each particular field is regulated not by the engineer, but by an automatic device connected with the growing crop and responding to its demands.

New York, Henry Holt And Company.
London, Williams and Norgate, originally published 1913