554 CRAWFORD AND DAVY ON THE 



is absent in insects, nearly all the lower orders 

 of invertebrated animals, and is confined to a 

 small proportion of the blood in fishes ; from 

 which it would appear, that colour is rather a 

 concomitant, than an essential or vital property 

 of the blood. 



In regard to the agency of caloric in producing 

 a change of colour, some interesting experiments 

 were performed by Crawford, who found that 

 after immersing a dog, with the exception of his 

 head, in water at 45 F. for fifteen minutes, his 

 venous blood was the darkest he had ever ob- 

 served ; but on placing another dog in water at 

 1 14 for thirty minutes, it was of a scarlet hue, 

 and could scarcely be distinguished from that of 

 a neighbouring artery.* (Experiments and Ob- 

 servations on Animal Heat, pp. 310, 387.) And 

 Dr. John Davy states, that there is much less dif- 

 ference between the colour of arterial and venous 

 blood of sheep when the temperature is 80 or 

 90, than during winter in England, or when the 

 mercury is below 32. (Physiological and Ana- 

 tomical Researches, vol. ii. p. 140.) In ac- 

 cordance with these facts, it may be observed, 



* He supposed that the dark colour of venous blood was 

 owing to the carbon it obtained during its circulation through 

 the systemic capillaries ; and that by elevating the temperature 

 of an animal above the natural standard, this change was pre- 

 vented from taking place. But the experiments of Hunter, 

 Hassenfratz, and others, demonstrate that arterial blood passes 

 into the venous state without either the loss of oxygen or the gain 

 of carbon. 



