THE DIGESTIVE SYSTEM 



225 



associated with the sucking mouth and cannot be compared with the 

 tongue of higher vertebrates since the hypobranchial muscles which form 

 the mass of tongue muscles in higher vertebrates, though present in 

 cyclostomes, have no connexion with the so-called tongue. 



Gnathostome fishes have an immovable tongue, which .forms a swelling 

 in the floor of the mouth and is supported by the basihyal or os ento- 

 glossum. Although it lacks muscles, this fish tongue is generally regarded 

 as homologous with the root of the tongue of tetrapods. 



The tongue of tetrapods, beginning with amphibians, consists of an 

 apex and root as in man. While the root is derived from the tongue of 



BODY 

 ''sft. TUBERCULUM 



TUBERCULUM' \ IV 

 IMPAR •■ 



EPIGLOTTIS 



Fig. 2 12. — Two stages in the development of tongue and pharyngeal floor of man. 

 The body of the tongue comes from paired and unpaired anlagen of the mandibular arch ; 

 the root from second and third visceral arches. That the fourth arch is involved is 

 doubtful. (After Kallius.) 



fishes, the body is a new formation derived from the mandibular arch 

 united with a median outgrowth from the floor of the mouth. 



The tetrapod tongue is further modified by the ingrowth of hypo- 

 branchial muscles by which it attains a high degree of mobility. Con- 

 sequently, in addition to its other functions of moving food in mouth 

 and swallowing, it serves as a means of capturing food. Its gustatory 

 function continues throughout the entire vertebrate series. Some have 

 assumed that the primary function of the tongue muscles was that of 

 squeezing secretions out of the lingual glands. Papillae appear first in 

 amphibians, but become more highly diiTerentiated in mammals. 



The Pharynx 



The pharynx is that part of the alimentary canal where the respiratory 

 and digestive passages cross one another. It is bordered by the soft 

 palate above, the tongue below, and the glossopalatine arch on each side. 

 The glossopalatine arch partially covers the palatine tonsil, a mass of 



