Terminal Nerve in Odontocete Cetaceans

摘要

Cetaceans evolved from land mammals that entered the seas in the early Eocene some 55 to 60 million years ago. During the course of their evolution as completely aquatic mammals, the external respiratory opening shifted to the top of the head to allow the animal to take a breath while swimming rapidly. Two openings (blowholes) are present in baleen whales and one in toothed whales. In dolphins such as Tursiops truncatus a single blowhole with its muscular plug sits atop a system containing three pairs of asymmetrical sacs, below which are paired respiratory passages, two nasal cavities separated by a bony nasal septum. Although there are no olfactory nerve endings, and much of the anatomical arrangement is unique to cetaceans, the paired external air passages and their associated cartilage, sacs, plugs, ligaments, and muscles are referred to as the nasal system.