GENERAL PATHOGENESIS AND PATHOLOGY OF CHILDHOOD exceed certain limits the stimuli become strong enough to produce a change in the cell. These changes are recognized by some disturbance of the normal physiology and are known as So long as the changes in the conditions of life do not produce disease or sickness they are considered physiological stimuli. When they pass this physiological point and cause disease the stimuli are said to be aphysiological or pathogenic.
The greater the variations in the environments of a cell or its fore bears, the stronger can lie the stimuli without making such a cell dis eased. On the other hand, a cell whose progenitors have always lived in uniform environments will feel a very slight change of the surround ings as an aphysiological stimulus. Thus the origin of a disease is al ways an aphysiological stimulus, one to which the cell is unaccustomed and against which it is not strong enough to fight.
It is comparatively easy to understand what are the best environ ments for a unicellular organism; what is a physiological and an aphysi ological stimulus, or what represents disease for the cell; but it is a much more complicated matter to find out the same things for the human body. This depends on the fact that each cell of the body is widely different from the others, and has a markedly different sensibility to the same disease. Each group of cells is further seen to be very dependent on the others, so that a slight change in the one will call forth at times a severe disease in the other. In this connection, attention should be called to the importance of the internal secretion of certain organs (the lymphatic nodes, adrenals, etc.).
The Importance of the Epithelium. The cells which play the most important part in the appearance of a disease are those found on the external and internal surfaces of the body.
The Epithelium. Coming into contact with the outside world these cells are naturally more accustomed to changes in their environ ment, and by their action maintain the normal uniform conditions necessary for the existence of the deeper cells. The most important
epithelial organs are the skin, the digestive system, atm the respiratory system.
The cells of skin are accustomed to fluctuations of temperature to which the deeper cells are not. Thus we see that the skin, which is able to obstruct the entrance of foreign bodies, eon also keep the tem perature of the internal organs at a uniform degree. It accomplishes this by certain mechanisms, but only so long as the variations of external temperature are not too great.
The epithelial cells of the digestive tract prevent the entrance of bac teria and foreign albumins, by means of their digestive power. Thus, in spite of their permeability, which they must have in order to absorb food, they form a protective apparatus of extraordinary value.
The cpithe/fa/ ce/is of the twigs allow the interchange of oxygen and carbonic acid gas, but cannot Of themselves prevent the entrance Of foreign bodies. Such subsitilleeS ;ire caught he the to and fro motion or the ciliated epithelium of the upper air-passages. Warming the air as it passes through the nose, larynx, and trachea is another duty of the epithelium, which thus guards organism from a too great reduction of temperature in the act of breathing.
Naturally, this function of the epithelium depends on eertain ex ternal conditions. So as the variations of these conditions are not too marked,—in other words, so long as Hie external temperature and the atmospheric pressure fluctuate bet ween certain limits, so long as the dust and bacterial content in the air is not too great, and so long as the nutrition is good,—the liodv will remain healthy. When the external conditions deviate to any extent and become aphysio logical, the epithelium becomes diseased and may even die. The epithelium will then be an insufficient protection, and substances can enter and do llama.