Page:Natural History (1848).djvu/110

100 single bone of inconsiderable thickness. Several species have a perpendicular fin standing up from the back, but it is merely cartilaginous, and never supported by any bony processes from the spine.

The modification of the respiratory organs requisite for animals whose whole life is passed in the sea is not the least curious point in the economy of the Cetacea. Formed for breathing air alone, and therefore compelled to come to the surface at certain intervals, they yet remain immerged for periods which would prove fatal to any other air-breathing animals. ‘The Whales can remain upwards of an hour beneath the surface. The object of breathing being to renew the vital qualities of the blood, by the absorption of oxygen from the air, it is manifest that if more blood could be oxygenised at once than is wanted for immediate use, and the overplus deposited in a reservoir until required, respiration could be dispensed with for a while. Creative wisdom has obviated the difficulty by this contrivance. The exhausted blood which is returned by the veins, having been renewed by communication with the air in the lungs, is carried to the heart, whence only a part is carried away into the system, the remainder being received into a great irregular reservoir, consisting of a complicated series of arteries, which first lines a large portion of the interior of the chest, then insinuating itself between the ribs, forms a large tata outside of them near the spine, and also within the spinal tube, and even within the skull. The blood thus reserved is poured into the system as needed, and thus frequent recurrence to the atmosphere is dispensed with.