EXPERIMENT STATION BULLETINS. 309 



the disease of tuberculosis, whether in the throat, lungs, abdominal cavity or any other 

 part of the body. Elsewhere we shall have something to say about the dissemination 

 of tlit'se tubercles in the body. 



What has just been said about tubercles is limited to the naked eye appearance. 

 To see into them still farther requires the aid of the microscope. The tubercle in its 

 youngest stage resembles a congested point in the tissues. There is an extraordinary 

 increase in tlie number of cells. With the constant increase, due apparently to some 

 irritant, the inner cells begin to disintegrate and the accretion is on the outer side. 

 This process continues until considerable tissue is involved, and the tubercles take on 

 their macroscopical appearances, which have been described above. 



A tubercle seems to be the result of some irritant at work in the tissues. What 

 that irritant was, years ago, proved to be a very difficult problem. Today we ascribe 

 the irritation to one thing only — the tubercle bacillus. By no means, should it be 

 understood that the contagiousness of tuberculosis is a matter of recent origin ; it 

 dates back into the past centuries when in many instances laws were enacted to isolate 

 consumptive individuals and to burn everything with which such persons had come in 

 contact. In fact, long before the tubercle bacillus had been discovered by Koch in 

 1882, its powers of producing tuberculosis were practically demonstrated by the inocu- 

 lation of tuberculous virus from a sick animal into a sound animal. 



What this tubercle bacillus is, can best be comprehended by referring to it as far 

 beyond the vision of the naked eye. It is a single cell instead of a multiplicity of 

 cells such as are found in the higher plants — a single cell, elongated so that it would 

 resemble a section of a lead pencil, rod-shaped, with rounded corners and very slender. 

 Its average length is about one-thirteen thousandth, of an inch. The naked eye will 

 resolve about two hundred lines to an inch, but put end to end, thirteen thousand 

 tubercle bacilli would be required to make an inch. Usually when found in sputum 

 or in tubercles these little rods or bacilli are slightly bent, giving them a characteristic 

 appearance. In the state of nature, they are translucent or resemble ground glass if 

 viewed in a drop of water. Occasionally they are found in little bunches, the indi- 

 viduals in which are arranged parallel to each other. At times there appear within 

 each cell small bodies which have been regarded as spores by some. Single spores have 

 also been attributed to these bacilli, but, while the spores remain questionably demon- 

 strated, the general verdict seems to accept spore formation as absolute. It is difficult, 

 for instance, to demonstrate tubercle bacilli in circulation and also in old broken 

 dowm tubercles by the use of a microscope, yet, especially in the latter, transmission 

 into a susceptible animal is easily made. Other forms of the tubercle bacillus appear. 

 The branched form has been noted by several observers, but this may be considered 

 rather unusual. For this reason it has been likened to the ray-fungus of lumpy jaw 

 (actinomycosis). It is easily understood, if the size of this micro-organism is con- 

 sidered, how the disease may be easily disseminated and conveyed from one person to 

 another. 



In this connection arises the question whether the tubercle bacillus of man and cow 

 is identical. Smitli has shown some cultural, morphological and patliological differ- 

 ences existing between tubercle bacilli coming from a human source, as sputum, and 

 those obtained from a bovine source. Although these differences exist, tubercle bacilli 

 taken from human sputum are capable of producing tuberculosis in catlle, yet the 

 progress of the disease is much slower than when the tubercle bacilli are secured from 

 a bovine source. It is possible these differences are founded upon the environments 

 of the bacillus in each case, and, as Smith states, they may be found in the diminished 

 susceptibility of man or a higher pathogenic power of the bovine variety. The study 

 of the tuljercle bacillus in man and animal comparatively may lead to far-reaching 

 results and may lead also to an elucidation of many features of tuberculosis which are 

 little or unsatisfactorily understood at the present time. 



It. dijcs not seem necessary at this time to demonstrate how it is proved that the 

 tubercle bacillus is the sole cause of tuberculosis: yet the question is frequentlj' put to 

 us, "How do you know that the bacillus is the cause?" If we sliould see a field of wheat 

 filled with daisies and the wheat itself was very light, we would at once suspect 

 that the cause of the wheat failure was due to the daisies. This would be sufficient to 

 satisfy many minds. But there are other possible causes for the failure of the wheat, 

 such as soil, moisture, climate, etc.. any one of which might l)e the cause. If every 

 poor wheat field we saw had numerous daisies, we would be more convinced than ever 

 that the daisies were the cause; yet soil, moisture, climate, etc., may be the cause so 

 far as any evidence indicates. However, if we sliould take a ])iece of this same land, 

 subject to the conditions of moisture and climate as above, and which we know 



