ANIMAL HEAT. 73 



dependent upon the entrance and exit of air, which effect the 

 gaseous exchanges. Taken strictly in this sense it is an incorrect 

 term, inasmuch as in the respiratory movements of animals pro- 

 vided with branchial cavities we have to do with the entrance and 

 exit of water. 



In the higher animals provided with red blood, the difference in 

 the condition of the blood before and after its passage through the 

 respiratory organs is so striking that it is possible to distinguish 

 blood rich in oxygen from blood rich in carbonic acid, by the colour. 

 The latter is dark red, and is known as venous blood ; the former, 

 i.e., blood which has just left the gills or lungs, on the contrary, 

 has a bright red colour, and is known as arterial blood. 



While the terms venous and arterial are used in an anatomical 

 sense to express the nature of the blood-vessel, those carrying the 

 blood to the heart being called venous, and those carrying it from 

 the heart arterial, they are ako used in a physiological sense as an 

 expression for the two conditions of the blood before and after its 

 passage through the respiratory organs, i.e., to express the quality of 

 the blood. Since, however, the respiratory organs may be inserted in 

 the course of either the venous or arterial vessels, it is obvious that, 

 in the first case, there must be venous vessels carrying arterial blood, 

 (Molluscs and some Vertebrates), and, in the latter, arterial vessels 

 carrying venous blood (Vertebrates). 



Animal heat. The intensity of respiration stands in direct relation 

 to the energy of the metabolism. Animals which breathe by gills 

 and absorb but little oxygen are not in a position to oxidise a large 

 quantity of organic constituents, and can only transform a small 

 quantity of potential into kinetic energy. They perform, therefore, 

 not only a proportionately smaller amount of muscular and nervous 

 work, but also produce in only a small degree the peculiar molecular 

 movements known as heat. The source of this heat is to be sought, 

 not, as was formerly erroneously supposed, in the respiratory organs, 

 but in the active tissues. Animals in which thermogenic activities are 

 small have no power of keeping independently their own internal 

 heat when exposed to the temperature influences of the surrounding 

 medium. This is also true of those air-breathing animals in which 

 the metabolic and thermogenic activities are great, but which, in 

 consequence of their small size, offer a relatively very large surface 

 for the loss of heat by radiation (Insects). On account of the ex- 

 changes of heat which are continually taking place between the 

 animal body and the surrounding medium, the temperature of the 



