178 THE STRUCTURE OF MAN 



explanation which, since it appears to rne 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 arteriae pulmonales (which, as fig. 1 5 in His's work shows, 

 descend quite symmetrically) come to lie 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. I 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." l 



1 I put forward these views of Hasse with all reserve, and I would draw attention 

 once more to a point already touched upon in dealing with the thoracic skeleton 



