RESPIRATION OF SEALS. 21 



unaccountably monopolized the attention of physiologists. 

 The theory I ventured to propose of this phenomenon of 

 suspended respiration in the Annals of Philosophy for 

 August 1827, is, " that the conditions of the nervous sys- 

 tem of aquatic animals are such, that venous blood requires 

 a much longer period to circulate through their cerebral 

 arteries than through those of land animals, to produce the 

 same deleterious effects.'" I have seen no reason in the 

 main to change this ; it may, however, be a little extended 

 to include the hypothesis that the venous blood of diving 

 animals may differ in its qualities from that of land ones. 



Seals, when on land, respire seldom, — perhaps every two 

 or three minutes. It seems as if too frequent respiration 

 induced a state of aeration in their blood, too excitable for 

 their systems, and productive of fever, causing in them a 

 state of disease not dissimilar to what we might experience 

 if the atmosphere contained more than its regular propor- 

 tion of oxygen. The beneficent Creator, who has assigned 

 to these animals so great a proportion of their existence un- 

 der water, has, we must believe, conjoined with this the 

 condition of enjoyment and of health, not of suffering and 

 disease ; and, in this view, we may find a reason why 

 aquatic animals may not long survive when the opportuni- 

 ties of diving are withheld from them. A sub-arterial state 

 of their blood may be necessary for the adequate performance 

 of their functions, and the equable and low temperature of 

 the ocean may be required to reduce the vital heat (which 

 in them is observed to be at a high standard), and which 

 may be mainly produced, not only by the great compara- 

 tive quantity of their red blood, but also by its venous 

 character, and the lesser use which their habits lead them to 

 make of the lungs, as a safety-valve for their superfluous 

 carbon and caloric. In the beats of the heart I do not re- 



