THYROID 



331 



is always extensile except in the Cetacea. The intrinsic muscu- 

 lature is highly developed, and may even extend backwards over 

 the sternum (Man is, Myrmecophaga). In the Ruminants, in which 

 upper incisors are wanting, it is very important in browsing. In 

 some cases (e.g. Felidae) its surface is horny. 1 A fold, the so-called 

 sublingua (plica fimbriata), is present on the lower surface of the 

 tongue, and is especially well marked in Lemurs ; in the Slender 

 Loris (Stenops) it is supported by cartilage. 2 This structure has 

 been supposed to correspond to the last vestige of the tongue of 

 lower Vertebrates (Reptiles) which has been replaced by the more 

 highly-developed organ characteristic of Mammals, the latter 

 having arisen secondarily from the reduced sublingua. 3 



THYROID. 



In Amphioxus, as in Ascidians (Tunicata), a ciliated groove, 

 the endostylc, is present along the ventral border of the extensive 

 pharynx, and the cells lining this groove secrete a glutinous 

 substance in which the food particles be- 

 come entangled and by the action of the 

 cilia are carried onwards to the intestine. 

 This endostyle is without doubt homologous 

 with the thyroid of Craniates, the presence 

 of which is as characteristic of them as is 

 the notochord. In consequence, however, of 

 the different method of taking in food and 

 the presence of jaws, it has undergone a 

 change of function and never remains open 

 to the pharynx throughout life, but gives rise 

 to a so-called " ductless gland," the substance 

 formed by its "internal secretion" passing 

 into the lymph or blood. 



The thyroid arises primarily as a median 

 ventral diverticulum of the pharynx which 

 extends along the region of the first four or Fio. 242. THYROID 



AND THYMUS 

 Lacerta agilis. 



OF 



five visceral clefts, and in the course of de- 

 velopment may become subdivided into two 

 lobes. In addition to this unpaired diver- ^eart; ^, trachea ; 



., ., . 'iii * "*' Lnymus ; j. T) 



ticulum, paired portions, situated more pos- thyroid. 



teriorly, are developed in Mammals : the 



former, as in all Gnathostomes, arise from the basihyal region, 



1 Horny papilla are present on the tongue of Ornithorhynchus. 

 -called 



2 The so-called "/y.ss-a" of the Mammalian tongue consists partly of cartilage, 

 and partly of muscle, fat, and connective tissue : it corresponds to a vestige of 

 the lingual cartilage of lower Vertebrates, and undergoes various modifications. 



3 According to another view, the sublingua is not a vestige, but a structure 

 appearing for the first time in certain Mammals which has been secondarily dif- 

 ferentiated from a ventral portion of the tongue proper : even then, however, its 

 cartilage may have an ancestral significance. 



