234 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



V 



the neighborhood of the nostril, a condition correlated with the use of 

 the tongue in these animals. 



Among the mammals, the whales excepted, the tongue is very 

 mobile, reaching the extreme in this respect in the ant-eaters. This 

 mobility is largely due to the intrinsic muscles, derivatives in large 

 part, from the hypobranchial musculature. The tongue is developed 

 from the unpaired tubercle (tuberculum impar), from two thicken- 

 ings on the mandibular arch (fig. 286) and its hinder portion from 

 fleshy ridges above the hyoid arch (fig. 247). The line between 

 anterior and posterior parts is largely obliterated in the adult, but it 

 is close to the circumvallate papillae (p. 204) and the foramen caecum, 

 a pit connected with the development of the thyreoid gland (p. 265). 



mth 



Fig. 247. — Two stages in developing tongue and pharyngeal floor of man, after His, 

 c, copula (basihyal element); cs, cervical sinus; ep, epiglottis; g, glottis; h, hj'oid arch; 

 md, mandibular arch; mth, median anlage of thyreoid; t, tuberculum impar; tg, tongue. 



Thus the mammalian tongue is similar to that of reptiles and exceeds 

 that of birds in having mandibular components. 



There are two views as to the relations of the mammalian and amphibian 

 tongue. In the one the amphibian tongue is unrepresented in the mammals, 

 unless it be by the sublingua, a fleshy fold beneath the tongue of marsupials 

 and lemurs (fig. 248), traces of which occur in other mammals, even in man, 

 as folds (plicae fimbriatae) beneath the tongue. In some cases (Stenops) the 

 sublingua is supported by cartilage, which may be the entoglossum (p. 106). 

 Others think that the amphibian tongue is represented in the anterior part 

 of that of the amniotes, and these regard the lyssa (a vermiform mass of 

 cartilage, muscle and connective tissue (fig. 249) lying ventral to the median 

 septum of the tongue) as the equivalent of the entoglossum and its associated 

 structures. 



The tongue varies considerably in shape in the different mam- 

 malian orders, but the differences are of little morphological impor- 

 tance. The dorsal surface is usually covered with a soft epithelium 



