178 THE STRUCTURE OF MAN 
explanation which, since it appears to me to possess a certain 
degree of probability, may be here recapitulated. He writes: 
“Since the heart and its immediate connections push the right 
primary pulmonary sac, which from the first is larger than the 
left, backwards and upwards, the branches of the fifth aortic arch 
—the arterize pulmonales—(which, as fig. 15 in His’s work shows, 
descend quite symmetrically) come to he somewhat differently 
on the two sides. The right artery must cut across and overlie 
the primary lung-sac earlier than the left, and become therefore 
the sooner connected with it. Herein, perhaps, also lies the 
explanation of the greater growth of the right sac, and of the 
fact that this gives rise to a special outgrowth, the foundation of 
the eparterial bronchial system. JI am the more inclined to this 
belief, and to that in the above-named determining causes, by the 
fact that in cases of situs inversus and reversal of the heart and 
great blood-vessels, the relationships of the right and the left 
main bronchi, and indeed of the two lungs as wholes, are also 
reversed (Weber, Leboucq, Aeby).” 
This is not the place to consider further either the relationships of the 
bronchial system, the differences in its distribution in relation to the planes 
of the body, or the changes which it undergoes after birth. For these 
details I must refer the reader to the original monograph. In the same 
work is to be found a discussion of the arrangement of the bronchial system 
in adult human beings, the explanation of which may be summarised as 
depending upon the direction of movement of the single points of the 
thoracic walls lying round the lung. Hasse concludes his interesting account 
as follows :—“If it be admitted that the tendency towards modification 
conditioned by the mechanism of the walls of the thorax is inherited, then 
we must allow that the facts point back to the form of lung of the earliest 
ancestors of Man among the Amniota, and to the changes which the respirat- 
ing organs have gradually undergone in the course of time in the ancestral 
series. The principal direction of the bronchi is at first downwards and 
backwards. From this it follows, it seems to me, that in the ancestors of 
Man the diaphragm first played the principal part in respiration. Then 
the system of branches running outwards and downwards is developed in an 
ascending degree. From this I conclude that thoracic respiration next super- 
vened in increasing degree, this being most marked in the lower, or better, 
the posterior part of the thorax, and least marked near its upper and anterior 
region. By degrees the upper and anterior part of the thorax took an 
increasing part in respiration, and this led to the mechanism of respiration 
which is illustrated in Man, This course of the development of respiration 
and of the respiratory movements, it appears to me, is in exact correspond- 
ence with the development of the respiratory organs as I have explained 
them, and with the facts brought to light by Aeby’s investigation of the 
bronchial tree of the lower animals.” ! 
1 T put forward these views of Hasse with all reserve, and I would draw attention 
once more toa point already touched upon in dealing with the thoracic skeleton 
