PULMONARY EVOLUTION IN MAMMALIA 197 
elaboration of the archeal lung-stem, retaining the primitive gen- 
eral plan of the early pulmonary organization. The differences 
in the biochemical energies of the two great groups are commen- 
surate to the changes in their respiratory tracts which are the 
morphological expression of these functional activities. Inter- 
preted from the standpoint of their response to the physiological 
demand, the structural gap between the lungs of the two higher 
warm-blooded classes and those of the cold-blooded Amphibians 
and Reptiles is not more abrupt or pronounced than is war- 
ranted by the difference in the coefficient of their performances. 
2. In estimating the influence of the environmental and func- 
tional factors which have led to pulmonary extension, especially 
of the cranio-ventral area, in the mammalia, a heightened ratio 
of respiratory metabolism forms the key-note. 
In general terms the degree of bodily temperature depends 
upon the process of oxygenation of tissue during periods of func- 
tional activity. The large glands and the muscles are the chief 
sites of this catabolic combustion. The lung represents, so to 
speak, the chimney or flue through which the gaseous products 
of this combustion are removed and the gaseous elements for a 
renewal of metabolism are introduced. -Greatly increased 
weight and volume of the entire body, coupled with rapid or 
long continued locomotion implies the use of a bulky muscula- 
ture and stipulates an active and abundant respiratory exchange. 
The morphological response of the lung to this factor will depend 
on the mode, force, and rapidity of the locomotion, and especially 
on the duration of the required muscular effort during a given 
period. The homely expression ‘loss of wind’ signifies morpho- 
logically and functionally a pulmonary equipment which finds 
itself unable to take care of the accumulating waste products of 
long continued muscular exercise. A man is really not ‘winded’ 
but ‘lunged’ under these circumstances. 
The influence of this functional aspect of respiration on pul- 
monary organization is seen in the mammalian ungulate lung, 
with extensive eparterial unfolding of the cranio-ventral seg- 
ments. It also plays a part in the production of the pulmonary 
extension of the large cetacean and sirenian forms, although 
