The reason why the different organs of the same species react dif ferently to the same disease is that in spite of their typical similarity to each other they arc differently constructed, no•cording, to their different functions. So we speak of an orp,p-disporlion. `I'o illustrate, the gonococcus produces ordinarily an inflammation of the conjunctiva, joints, Auld lower urinary passages: the diphtheritic organism affects the tonsils and upper air-passages; while the specific cause of mumps produces an inflammation merely in the parotid gland and its immediate vicinity. These phenomena depend, we imagine, on a rather definite selective power, the actual existence of which is still undentonstrable. It is supposed to be a power of attraction existing between the 1)acteria, and their toxins for the cells especially affected by these organisms.
While the cells of the different species, as also those of the different organs, have a slight variation one from another, we see that they also differ according to the age of the individual. This idiosyncrasy of age, intimately connected with the idiosyncrasies-of race and organ, helps to explain why the cells of a child react to a stimulus in quite a different manner from those of the adult.
The normal disposition we consider to be that peculiarity of the healthy individual to respond to a definite etiological cause by the ap pearance of a particular disease. If we believe this, then it must naturally be concluded that there exist constitutional tendencies more or less different from the normal, which, reacting to a stimulus normally phys iological, produce a disease. This increased susceptibility to a disease is influenced by external conditions, not only those which aid in its appearance, but also those which have an unfavorable influence on the course of the disease. Such hypersensitiveness to special diseases may be inherited, or may be acquired from unfavorable hygienic conditions, dependent on poverty, mistakes in feeding, and other diseases already existing. A single harmful influence may of itself be unable to produce this condition, whereas in combination with others, disease would readily appear.
There are many things which in a general way help to answer the question as to why certain disorders often appear in childhood, or at least seem more prominent at this time. Acute infectious diseases, for instance, seem to predominate in childhood because the adult is immune, having already experienced them. Again the imperfect development of certain organs in the child, as compared with the same organs in later life, offers a reason for the frequency and different course of certain other diseases seen at this time. Not only is the period of childhood a
factor in disease, lutt the period of adolescence also and certain outside condifious as well Acute Infectious Diseases. We will speak first of the acute infectious diseases peculiar to the child, by the laity called the "diseases of childhood." Most infectious diseases are more frequent in childhood because, in contrast to the adult, children possess an especial suscepti bility, (Inc to their age. The fact that the first attack of these diseases usually makes a person immune, and the fact that children are espe cially to them. explain still further why they appear much oftener in childhood than in later life. This frequency depends not only on a disposition of age but on a racial disposition; that is, the fact that all mankind is susceptible to these diseases. To this group of diseases belong measles, ri5theln„ scarlet fever, and whooping-cough.
The tendency to catch scarlet fever and whooping-cough, while it is less in the adult than in the child, still remains throughout life. In contrast to this we see that the peculiarity to take chicken-pox and mumps, so marked among children, is not seen in adult life, although the individual may not have experienced them during earlier years.
All the diseases above mentioned, with a rare exception, give pro longed immunity; that following diphtheria, however, is quite often of short duration. It is not known whether this depends on a disposition peculiar to the individual or on an abnormally rapid disappearance of the antitoxin.
The origin of acute infectious diseases is the presence of the con tagium or exciting cause, which remains on the mucous membranes of the mouth, pharynx, and throat of the child. If present in sufficient amount this contagium, after its incubation period. produces the dis ease characteristic of it. Every infectious disease has a more or less definite type of infection, which depends, first, on the peculiarity of the exciting cause, and second, on the disposition of the patient. To illustrate: in measles the contagium is supposed to live outside the body only a very short time, so that the rapid spreading of on epidemic can only he explained by the extraordinary high susceptibility of man. Its transmission almost exclusively arises from direct contact. Quite the opposite is seen in scarlet fever. The exciting cause in this instance re mains active for a long time outside the body and is still able to produce infection. So that, in contradistinction to measles, the susceptibility for scarlet fever is not so great.