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 tonsils. 



cr'op 



$l:omocc"h 



.gi3)5ard; 



intestine 



Stomach" of bird. What are the 

 functions of the different parts .^ 



