Another factor must be considered in relation to the appearance of tuberculosis; namely, what is called by Behring the defect" healing of the tubercle. When the tuberculous lesion is entirely healed. instead of an increased resistance against tuberculosis being left, we see a greater sensitiveness remaining. A tuberculous lesion may disappear entirely, but it accordingly leaves in the individual a susceptibility for the tuber cle bacillus, which on a second infection by air, produces a slow-healing tubercular process.
In this connection, liaumgarten's theory of placental infection is important, especially so, as Schmorl and Geipel have for the first time shown that the possibility of placental infection is more frequent than supposed, finding as they do tubercular changes present in the great majority of placentas taken from tuberculous mothers. If a single focus heals, but leaves behind it a heightened sensitiveness for tuberculosis, then placental infection can play as important a role in the pathogenesis of tuberculosis as can intestinal infection. These ideas are, however, nothing hut plausible theories.
We have seen how important the welfare of the epithelial coverings is for pathogenesis in general and especially for that of the infant. We also learned, in speaking of infectious diseases, that beside the entrance of the injurious particle, the so-called disposition of the patient is most important in the genesis of the disease. In other words, it is necessary for the invading substances, usually bacilli and their poisons, to meet cells capable of reacting. When this fails, disease will not appear. Under these conditions a natural immunity, as we say, against the in vading disease is present. When bacilli enter an organism and produce a reaction, expressed by a certain disease, the symptom-complex char acteristic of this disease will take a certain course.
The sum total of the symptoms, which are nothing more than the result of the reaction between the exciters and their products on the one hand and the human cells with their products on the other, represent to us the course of the disease. Its study, Pathology, will in its narrow sense occupy us later on.
The etiological factor of many diseases is at the present time known. For ninny others, this factor is merely a hypothetical one, worked out from cause and effect. The idea of an organic cause for the effect has never been proved. However, in explaining such diseases, we must accept at present the possibility of such a cause existing.
Rachitis.--To this latter group of diseases belongs one of the most important diseases of childhood, rachitis. The present idea, in
contrast to that of older times, is that this disease is an affection of the entire organism, instead of the joints alone. We do not know the ex citing cause necessary for the appearance of rachitis. We only know that the origination of rachitis, as well as the severity of its course, is favored by circumstances \vhich have a general influence on the course of every disease. Nobody has been able to offer a satisfactory explana tion for its cause or origin; even the most careful and thorough investi gations of Pfaundler on the deposit of calcium salts in the animal tissues have proved of no avail.
IC is most probable that rachitis is not a disease ()I' the hones alone, but an infection of the entire organisni, an idea which is persistently held at the present time. It appears to lie a general disease, localized at the juncture of the cartilage iind bone as well as at the epiphysis. This is explained by the fact that at these places the greatest activity and proliferation of cells take place. In oilier words, they are the places where the life of the hone is most active. The growth of every organ in the child goes on symmetrically, while that of the bones ap pears centralized. Tliis growth does not take place throughout the entire hone, but seems to occur in certain peculiar zones, the cells of which have an especial power of development. The fact, that this affection picks out these spots especially, finds its analogy in the fact that syphilis seems to choose with predilection these same places as well, while in the older child and the adult such is not the ease. Rachitis depends therefore on the peculiar predisposition of the bones, and also on the apparently increased susceptibility at this point of most vigorous growth.
Rachitis occurs in Germany apparently endemically. The child otherwise absolutely sound, who has been fed on the milk of a healthy mother under the most hygienic conditions, is as little immune against rachitis as is the artificially nourished baby of the lower classes. The course of rachitis, however, will he found to be usually a light one in the former, and in the latter often severe. Kassowitz also emphasizes the importance of unfavorable hygienic environments in the appearance of rachitis, calling it "respiratory poison." The Importance of Social Conditions. — This is a good time to show the importance of the social status (i.e., the circumstances of the parents) in the appearance of diseases amongst infants.