CIRCULATION OF THE BLOOD. 19 



oxygenated blood contained in these canals to support the 

 action of the systemic heart, and supply the arterial circu- 

 lation, during the long period in which breathing is sus- 

 pended, tith, That venoid blood is equally an immediate 

 cause of asphyxia to aquatic as to land animals, and more 

 generally that what we observe to act as a poison on the 

 one class within a certain time, must necessarily do the same 

 to the other ; and farther, that the venous blood is the 

 same in quality in the aquatic as in the terrene mammalia. 

 We may not have ascertained the use of this distribution 

 of arteries in these diving animals, but this is no reason why 

 we should conclude that it must be connected with the res- 

 piratory function. Peculiarities of structure may be found 

 to accommodate trivial changes of distribution of the blood, 

 but the question still returns, Whence is derived the arte- 

 rialized blood which is presumed to be necessary for carry- 

 ing on the active functions of the animal during so long a 

 period of suspended respiration ? Are we to suppose that 

 the cycle of the circulation is during this period nullified, 

 and that the blood in the arteries and veins remains oscil- 

 lating, without either fluid encroaching on the boundaries 

 of the other ? But even this fancy is negatived by the 

 fact that, in the case of seals being instantaneously killed 

 immediately after long submersion, I cannot say that I 

 could detect any arterial blood in the body ; throughout, 

 the vital fluid was dark, almost like tar, in the arteries and 

 left ventricle equally as in the veins. But why should we 

 persist in looking to structure as the only source of modifi- 

 cation of function ? Might it not be as easy to assume that 

 the venous blood of diving animals, when it enters the left 

 auricle, is somewhat different in quality from that of land 

 ones ; and that other organs than the lungs have previously 

 deprived it of its more immediately deleterious principles ? 



b 2 



