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Show 408 MR. R. BROWN ON THE SEALS OF GREENLAND. [June 25, find the foramen ovale unenclosed to within a line of the aorta. That of Blumenbach and Houston has been also brought forward, viz. that venous sinuses are to be found in the liver and surrounding parts, and that the large veins have been observed to be enlarged and tortuous; these have been supposed to act as reservoirs for the returning venous blood while the animal is diving under the water. But this theory carries inconsistency in itself. The venous system on the whole, and not in any particular part, unless in the vena cava, from the pressure excited on its walls, is greatly enlarged ; but this arises from the great quantity of blood these animals possess. But, even supposing these venous sinuses and that the animal will remain below the surface for twenty or twenty-five minutes (though I must assert that I never saw them remain longer below the surface than fifteen minutes, and from five to eight is the common time), are these sinuses large enough to contain the full quantity of blood that may return in that period from the capillary system ? The reply is certainly in the negative. Does the heart's action diminish in rapidity or come to a full stop ? in that case there would be no need of these sinuses. What, then, are the uses of them ? After a very careful examination, Dr. Wallace informs me that he never could find them, in all the Seals which he examined. He certainly remarked the dilated condition of the veins, but referred this to a physiological cause, viz. the pressure of the superincumbent column of blood. He believes that their power of remaining so long below the surface of the water is to be referred to a cause physiological, and not structural. Their expertness in swimming is not possessed from birth, but only developed from an innate instinct. He has often watched young Seals taking the water at first in smooth pools among the ice, and then swimming slowly and quietly about in the still floe-water-then gradually taking the water, staying below the water at first but a short time, gradually lengthening their stay until they had acquired the faculty of remaining the usual time beneath the surface. Dr. Wallace, then, thinks that this faculty is owing to a cause more physiological than anatomical, and that the explanation he has given, coupled with the enormous quantity of blood which the Seal contains, will account for their power of remaining beneath the water. As I have not examined the anatomy of the Pinnipedia with this object in view, I cannot presume to give an opinion on the matter; in the Narwhal and other Cetacea which I examined, the extensive venous plexus about the vertebral column seemed to explain the possession of this power of temporary subaquatic existence. The flesh of the Seal is quite black, from the enormous quantity of venous blood it is impregnated with ; but if exposed to the air or steeped in water, it acquires the usual arterial rosy hue. The flesh of young seals which have not yet taken the water is, on the contrary, quite red. 3. Habits and Instincts of Seals in general. They spend a considerable part of their time in feeding, but they |