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Rh microscopic portion of such organ or tissue. Even an acute disease—pneumonia, diphtheria—involves the body as a whole; chronic defects—such as heart lesion or cancer—affect the organism likewise in its entire extent. The pathologist, then, seeking to convey to the student an objective conception of the nature and effects of disease as a process, needs the entire body in order to do so. Pathology is taught for that purpose; it fails of its object just so far as the lack of autopsies makes it impossible. Cancer, for example, is not a local disturbance involving this or that organ. The student who is expected to grasp its character cannot do so if all he does is to see a cross-section put up in gelatine, or to handle a papier-mache reproduction, or to observe the cell changes on a small slide. These things are well enough as far as they go, but they go only a short distance. The cancerous process is complicated and extensive. Other organs, far from the original site of the disease, are involved; nay, the original site itself may be in question. The vastness of the involvements, the relationships of affected locations to each other, the response of the bodily mechanism fighting to achieve a readjustment—only the autopsy can disclose these; and without them, the student cannot attain an intelligent conception of the subject he is studying.

Pathology's greatest contribution to the comprehension and mastery of disease has been by way of illuminating its causation,—or etiology. The student who is to comprehend the significance of disease must not only make the inventory of results disclosed by the post-mortem: he should be allowed to observe the process from the very start. To this end, a demonstrative course, using living animals, must be provided. Tuberculosis, for example, should be exhibited through the inoculation of a few guinea pigs with different varieties of the tubercle bacilli, showing the various ways in which the bacilli enter and are distributed, and the variety of lesions that they produce.

We thus cross the threshold of still another science, bacteriology, developed in late years in close sympathy with pathology. It presents the same two sides,—biological and chemical; the former investigating the life-history of the microscopic organism, the latter isolating and resolving its toxic or other products. The search of the pathologist for the original causation of abnormal structural change has been immensely facilitated by the bacteriologist. He can now account for as well as describe the ravaged tissues that mark the path of a diphtheritic, typhoid, or tuberculous infection. Out of the life-history of the parasites in question has sprung the serum therapy, which has already stripped tetanus, diphtheria, and meningitis of much of their horror.

Perhaps even more important than its services to curative, have been the suggestions of bacteriology to preventive, medicine. It is hardly too much to say that modern hygiene, largely the outcome of bacteriology, has elevated the physician from a mainly personal to a mainly social status. Directly or indirectly, disease has been found to depend largely on unpropitious environment. A bad water-supply, defective