THE RESPIRATORY EXCHANGE 491 



The above data for the respiratory exchange have been ex- 

 pressed in relation to one kilogram of body-weight, but as 

 far as possible the weights of the animals have also been given. 

 The physiological unit, it cannot be too often insisted, is the 

 animal, and there is some danger of misunderstanding when the 

 results are always expressed in terms of one kilogram of body- 

 weight. The ratio of surface to mass is in the case of warm-blooded 

 animals a far more important factor, for, as later data will show, 

 it throws light upon the relatively greater respiratory exchange 

 of the small mammals and birds as compared with the bigger 

 members of the same genera, and the more rapid respiration of 

 the child or young animal as compared with the adult. In addi- 

 tion, an examination of the above table shows that the respiratory 

 exchange of most of the cold-blooded animals is very small, but 

 that in the case of insects the metabolism is most active and often 

 equals that of the larger mammals. This interesting exception 

 to the general rule is to be associated with the high temperature 

 and great muscular activity of insects. A comparison of the data 

 for birds and mammals will show that as a general rule the former 

 have a more rapid respiratory exchange, and this difference is 

 also associated with a higher bodily temperature. 



It is unnecessary here to discuss all the conditions which 

 influence the respiratory exchange, but there are some which 

 are of fundamental importance ; such are the age and size of the 

 animal, the influence of muscular activity, food, and external 

 temperature. 



Influence of Age upon the Respiratory Exchange. The process 

 of respiration in the embryo and foetus will be discussed in a later 

 portion of this article ; here the question to be considered is the 

 total intake of oxygen and output of carbon dioxide. In the 

 case of mammals there are great technical difficulties in the 

 estimation of the respiratory exchange of the foetus, for incision 

 of the uterus interferes with the circulation of the blood through 

 the placenta. It is probable from the few data which exist that 

 the respiratory exchange in the foetus towards full term is weight 

 for weight equal to that of an adult, for, although its muscular 

 activity is not great and its capacity to regulate its temperature 

 is imperfect, it is rapidly growing and is kept warm by the heat 

 of its mother's body. In the case of birds the respiratory exchange 

 can be studied under favourable conditions during the develop- 



