18 SEALS OF THE SHETLAND ISLANDS. 



tic birds is a subject of great interest, and has given rise to 

 much discussion. It has been stated, especially by Bichat, 

 that the venous blood becomes a poison to land animals 

 when it reaches the brain through the arteries. If, in the 

 case of seals, it does not circulate through the cerebral ar- 

 teries, some peculiarity of structure must exist sufficient 

 to explain so great an anomaly in the course of the blood 

 from what we observe in the other mammalia, and we have 

 also a right to expect to find some arterial blood in the ves- 

 sels after long submersion. The principal structural ap- 

 pearance that anatomists have hitherto detected, which they 

 suppose has reference to this point, is an enlarged capacity 

 of the abdominal veins ; but this, it is obvious, is totally 

 inadequate to account for the phenomenon. The use of 

 this provision is to be rather sought in its adaptation to 

 varying degrees of pressure to which the animal is exposed 

 in descending to different depths, and to its facilitating 

 especially the different states of contraction to which the 

 lungs may be subjected during submersion. To a similar 

 purpose, may not unlikely be referred that peculiarity of 

 circulation in some of the arteries of the chest, spine, and 

 cranium which John Hunter and other anatomists have de- 

 scribed in the cetaceae, for, on the supposition that these 

 ■plexuses of arteries exist in all aquatic animals, and that 

 they form a reservoir of aerated blood to supply the 

 exigencies of the circulation during the time that respira- 

 tion is suspended, we must be prepared to shew, 1st, 

 That there is a machinery and a power presiding over it, 

 by which the blood is retained in these arteries, or poured 

 from them into the cerebral circulation as its necessities 

 may require. 2d, That the venous blood is prevented from 

 entering the left ventricle, and thence flowing through these 

 vessels, as well as through every other ramification of the 

 arterial system. 3d, That there is a sufficient quantity of 



