. Biology; the story of living things. THE METABOLIC MACHINERY OF ANIMALS 287 the pulp cavity where during the Ufe of the tooth nerves and blood vessels are housed. Each tooth is held in a socket of the jaw by means of another hard tissue, the cement. Nearly every vertebrate organism possesses some sort of tongue which serves a variety of functions. The lassoing tongue so char- acteristic of certain amphibia, for example, is provided with special glands secreting glutinous mucus that helps to ensnare insects. In lizards the tongue may become extremely long and extensile, it also servii^ig to a
. Biology; the story of living things. THE METABOLIC MACHINERY OF ANIMALS 287 the pulp cavity where during the Ufe of the tooth nerves and blood vessels are housed. Each tooth is held in a socket of the jaw by means of another hard tissue, the cement. Nearly every vertebrate organism possesses some sort of tongue which serves a variety of functions. The lassoing tongue so char- acteristic of certain amphibia, for example, is provided with special glands secreting glutinous mucus that helps to ensnare insects. In lizards the tongue may become extremely long and extensile, it also servii^ig to aid jn capturing food, while among some of the birds it may even be adapted for impaling insects, as in the case of the "horny, spearlike tongue" of the woodpecker. The mam- malian tongue is likewise specialized, for in many of the herbivores it is definitely muscular and prehensile, being used to grasp tufts of grass which are then cut off against the lower incisors, while in dogs and cats it is used as a spoon to take up liquids. The tongue helps me- chanically in swallowing and in man it also plays a vital part in speech. The tongue of higher forms is covered with a variety of sensory structures which test the various foods before they are swallowed. The Phaeynx and Esophagus. This region is both membranous and muscular. We may think of the pharynx in all air or land verte- brates as being an irregular cavity supplied with openings. Dorsally and anteriorly are two posterior nares, or internal nostrils, laterally the openings of the Eustachian tubes connecting with the middle ear, while medianly and ventrally lies the opening to the oral cavity. Posteriorly there are two openings, one down the esophagus and the other, the glottis, leading into the trachea (see fig., page 285). Above the soft palate is a mass of lymphoid tissue, known as the adenoids, or pharyngeal tonsils, while anteriorly and laterally lie the true, or palatine cr'op $l:omocc"h .gi3)5
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