Page:Natural History (1848).djvu/111

Rh The windpipe does not terminate, as in other mammals, in nostrils at the extremity of the muzzle, but in an orifice at the very summit of the head, which, as the animal rises obliquely, is the first part that emerges from the surface, so that the admission of air to the lungs takes place without needless effort, or exposure of the body. The orifice, or orifices, for there are sometimes two, are called blow-holes, the expulsion of the long-imprisoned and heated air being accompanied with considerable noise, and with the ejection of water or steam. Cuvier thus explains the latter circumstance. "Let us suppose the Cetacean to have taken into its mouth some water which it wishes to eject. It moves its tongue and jaws as if it were going to swallow it; but, closing the pharynzx, it torces the water to mount into the nasal passages, . . . until it raises the valve (between the nasal passage and two pouches or reservoirs), and distends the membranous pouches above. The water once received into these pouches can be retained there until the animal wishes to spout. For that purpose it closes the valve, to prevent the descent of the water again into the nasal passages below, and forcibly compresses the pouches by means of the fleshy expansions which cover them; thus compelled to escape by the narrow crescentic aperture or blow-hole, it is projected to a height corresponding with the force of the pressure."

As the Cetacea descend to unknown depths in the sea, where the pressure of the incumbent water must be immense, the opening of this passage into the lungs requires to be guarded by a valve of no ordinary power. It takes the form of