THE DIGESTIVE SYSTEM 



305 



mandibular arch. The root of the tongue arises from portions of the 

 second, third and fourth visceral arches. The tongue muscles, however, 

 are not formed from those of the visceral arches, but from post-occipital 

 myotomes which send buds downward and forwards into the tongue. 



History of the Tongue. None of the lower chordates has a tongue, 

 so that the vertebrate tongue seems to be an emergent organ like the 

 notochord. The so-called tongue of cyclostomes is a muscular piston 

 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 connection with the so-called tongue. 



BODY 



"^ FORAMEN 

 CAECUM 



Fig. 256. — 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.) 



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 amphibia consists of an apex 

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

 amphibia, but become more highly differentiated in mammals. 



