If the infection follows according to the second and third methods, the primary focus is in the mesenteric lymph-nodes. This focus may be very small, not even macroscopic, and nevertheless form the start ing point, of a severe general tuberculosis. Indeed I have seen cases.
where the microscopic examination or even animal inoculation of the lymph-nodes was necessary to show the presence of tuberculosis.
There is one more possibility and one which has received too little attention, namely, that the bacilli can pass not only the intestine but also the mesenteric lymph-nodes without causing any lesions. We must remember that during digestion a strong current flows from the lumen of the bowel, and thence, traversing the intestinal wall, enters and passes through the mesenteric lymph-nodes. It does not seem unlikely that the tubercle bacilli, attaching themselves to the fat globules, may pursue this same course through the distended lymph spaces and obtain an entry into the thoracic duct, thence into the venous blood stream and right auricle. From here they are transported to the capillaries of the lungs where the blood stream is slower and then enter the lymphatics. They next find their way to the bronchial lymph-nodes, which become the primary seat of the disease. When the lymph-nodes cannot receive any more foreign material, dissemi nation takes place in the lungs. Animal experimentation shows that the injection of tubercle bacilli into the jugular vein leads to a tuberculosis of the bronchial lymph-nodes and lungs. Also, that after giving tuber culous food, the capillaries of the lungs are full of tubercle bacilli.
Two points must be especially considered. First that the diges tive apparatus does not commence at the stomach, but at. the lips; so that at any point of the intestinal tract, from mouth to anus, infection may occur. And also that there are certain points which are favor able portals of entry for the bacilli. In young children the mucous membrane of the mouth is a locus minoris resistentire (of course not on account of dentition) as are also the pharyngeal and faucial tonsils, which through their crypts afford an excellent resting place for the bacilli. Whereas the stomach and small intestine appear rarely to be the seats of attack of the tubercle bacillus, the large intestine is frequently invaded. We may add that all bacterial infections attack this section of the intestine more readily than any other.
The tubercle bacillus may cater any part of the intestinal tract. In fact, under certain conditions it may gain admittance to any other part of the surface of the body, whether it be covered with mucous membrane or skin. According to our present knowledge, we may say that. any part of the human body may at some time serve as the portal of entry for the tubercle bacillus. But as to the question of the chief mode of infection, our knowledge is by no means precise. This being the case, we must not direct our efforts of prophylaxis to one point alone but must consider the manifold ways in which a child may be exposed to tuberculosis.
Pathological tubercle bacillus having gained en trance into the body in one way or another, three possibilities present themselves. The organism may conquer the bacillus and thus pre vent the development of disease. This happy event may be brought about by the lessened virulence or small quantity of the infecting microorganisms, or by the strong protective powers of the child.
Second, the bacilli may remain in the tissues without causing disease or increasing in number. After a long period such bacilli, becoming more virulent through weakening of the protective agencies of the body, or through a favorable symbiosis with some other germ, may increase in number and give rise to the dread disease.
The third possibility is that immediately, or soon after entry into the body, the bacilli bring about the anatomical changes consti tuting a primary tuberculous lesion.
The tubercle, which gives its name to the disease, is a circum scribed inflammatory growth. It may be so small as scarcely to be perceived with the naked eye, or it may grow to the size of a millet seed or a pea. Its color is grayish yellow. Histologically it consists of closely packed epithelial cells, among which may be found giant cells, especially toward its centre. The tubercle bacilli are either extra cellular or intracellular. Characteristic of the tubercle is the lack of blood vessels. The tubercle is not caused by the vital activity of the bacillus, as dead bacilli or even foreign bodies may give rise to it. However, the further changes that the tubercle undergoes are due to the activity of the bacillus. They are the cause of the rapid central necrosis, the degeneration that transforms the nodule into a cheesy mass.