198 Proceedings of the Royal Physical Society. 



must always be in contact with fluid. In the lower forms 

 of animal life the habitat is aquatic, and the medium with 

 which the respiratory exchange takes place is the water. 

 The more complex animals, on the other hand, are charac- 

 terised by increase in bulk, so that their inner portions are 

 far away from the outer medium. In these we find that 

 watery fluid is formed in the interior of the creature, filling 

 chinks or more or less regular cavities. It is with this 

 internal fluid — this internal medium, as it has been aptly 

 termed — that respiratory change now takes place. There 

 will obviously be a tendency for the internal medium soon 

 to be robbed of all its available oxygen, and to become 

 loaded with carbon dioxide, so as to be useless for respiration. 

 To avoid this, we find (1) that a mechanism arises by which 

 the internal fluid is pumped along so as to circulate through 

 its series of chinks, in other words, a blood vascular system 

 arises; and (2) in one or more regions a highly vascular 

 surface is formed, which comes into intimate relation with the 

 external medium, the respiratory organ in the ordinary sense. 

 There is now a definite blood vascular system, and one, or 

 more than one, definite organ of respiration. 



It is clear that the actual process of respiration is of a 

 double nature. There is, firstly, what we may call tissue 

 respiration, consisting of the respiratory exchanges between 

 the living substance of the tissues and the blood; and 

 secondly, the respiratory exchanges, that take place between 

 the blood and the external medium. It is with the latter 

 of these two that what we ordinarily understand by respir- 

 atory organs have to do, and it is my purpose in this section 

 to trace what we may consider the most probable course of 

 the evolution of such organs amongst the Vertebrata. 



It may be assumed, to commence with, that the primitive 

 potential respiratory organs in vertebrates, or any other of 

 the more complicated Metazoa, are two in number — (1) the 

 skin, and (2) that part of the original outer surface which 

 has been tucked into the interior of the body to form the 

 alimentary canal. In each we have a more or less richly 

 vascular surface, which is brought into intimate relations 

 with the external medium as a whole, or with isolated 



