542 TRANSACTIONS OF THE CANADIAN INSTITUTE. [Vou. VIII. 
Our half-human progenitor very probably in the early stages of 
evolution suffered to the same extent from tuberculosis as other mammalia, 
but as soon as he took to the cave dwellings he became exposed to con- 
ditions which at once preserved the infected individual and also led 
to infection of other members of the family or tribe, and a more certain 
preservation of the infecting organism with a more rapid passage from 
individual to individual which we know from experimental work must 
have had its influence on the bacillus. With the increase of knowledge 
which led to the: domestication of the large herbivora and especially 
with the development of the use of milk as a food, there not only came 
the influence of the spread of tuberculosis among the domestic animals, 
but a continual re-infection of man from what we may presume was the 
parent mammalian bacillus. 
But there is another interesting character in human tuberculosis 
which is explained solely by evolutionary hypotheses.. Man may suffer 
from tuberculosis of any organ of the body, but the organ which is most 
frequently affected, and affected to the greatest degree, is the lung. Lung 
infection occurs in other mammalia and the lung probably is an organ 
which in itself presents favourable soil for the development of mammalian 
tuberculosis, just as the liver in birds is the favourite site of avian tubercu- 
losis. But man is peculiar in that one particular portion of the lung, 
viz., the upper or apical portion, is first affected. A variety of explana- 
tions are given of this, but the most plausible is that on account of the 
structure of the human skeleton and the position of the apex of the lung 
in the thoracic cavity, these upper portions are less completely ventilated 
and less richly supplied with blood. They form ‘‘loct minoris resistentie”’ 
in which the bacillus may unfold its disease-producing characters with 
better results. But there is a still more interesting point in regard to the 
lung. In man alone the first rib shows a greater tendency to develop 
fixity of position than in any other animal. All the ribs are joined to the 
breast-bone, or sternum, by cartilages which give the great elasticity to 
the thorax and allows of the proper movement of the lungs in respiration. 
In very old people these cartilages are apt to become stiff and calcified 
with a resulting interference with the freedom of respiratory movement. 
Freund showed many years ago that in tuberculous people, or in those 
belonging to families which show a tendency to develop tuberculosis, 
there is an early stiffening or calcification of the cartilage of the first rib 
with a resulting interference with the movement of the under-lying lung 
tissue. This earlier calcification of the first rib is not seen among other 
mammals not even among anthropoids, and it is probably one of the 
results of our upright carriage, which has produced altered mechanical 
conditions about the shoulder joint. We thus have a dysharmony arising 
