THYROID GLAND. 225 



in Mammals, it is very uniform both as regards the extrinsic 

 muscles and their attachment to the hyoid and lower jaw, and the 

 arrangement of the intrinsic muscles. Only in such forms as 

 Myrmecophaga and Dasypus do the intrinsic muscles show a 

 peculiar character : in them a tendinous investment is present 

 lying under the rnucosa, and this serves as a point of attach- 

 ment for all the transverse and vertical muscles. 



In many Mammals (e.g. Carnivora, Insectivora) a peculiar struc- 

 ture, the lytta ("worm"), is present, lying within the longitudinal 

 axis of the tongue. It is partly fibrous and partly muscular, and its 

 thread-like posterior end is connected with the body of the hyoid. 

 Its phylogenetic meaning is not yet clear, and it can only be said 

 that it functions as a point of origin and insertion for the intrinsic 

 muscles of the tongue. 



In Man the tongue has a double origin, arising firstly from an unpaired 

 protuberance the tuberculum impar (His) lying on the floor of the 

 mouth of the embryo, and secondly from the swellings of the second and 

 third visceral arches, which meet together in the middle line. In this manner, 

 a part of the floor of the primary mouth becomes bridged over, forming a 

 narrow depression, covered by the root of the tongue. This depression 

 becomes further isolated by the approximation and union of the body of the 

 tongue which arises from the above-mentioned tuberculum impar to the 

 tongue-root, which is formed from the visceral swellings. From the cavity 

 thus shut off a double epithelial vesicle is formed, the median thyroid 

 rudiment. This remains for a time in free communication with the cavity 

 of the mouth by a duct, the ductus thyreoglossus, which passes to the 

 surface of the tongue along the region where the body and root of the tongue 

 become later united. The foramen caecum, which can be occasionally traced 

 in the adult human subject into the substance of the tongue for 2 centimetres 

 or more, is the last remnant of this ductus thyreoglossus. From the median 

 thyroid rudiment the middle lobe of the thyroid arises later, and this com- 

 monly extends forwards (upwards) as a " cornu medium " and sometimes also 

 as a hollow duct. Now and then this duct may become constricted into 

 definite vesicles (from two to four), which are known as bursa suprahyoidea, 

 bursa prehyoidea, &c. All these are remains of the ductus thyreoglossus. The 

 lateral thyroid rudiments of Man arise by the lower part of the 

 primary floor of the pharynx, lying near the glottis, becoming separated off 

 from the main cavity, and thus forming an independent structure lying 

 laterally to the larynx. The lateral and median rudiments of the thyroid 

 later become approximated. 



Thyroid Gland. 



The thryoid gland arises in all Vertebrates as one or more 

 diverticula of the ventral wall of the pharynx or floor of the 

 mouth. In Ammocoetes the single diverticulum remains in com- 

 munication with the pharynx, and a similar condition of things 

 is seen in Ascidians and Amphioxus ; it thus appears probable 

 that we have to do here with a very ancient glandular organ, the 

 secretory function of which in relation to the alimentary canal was 

 of great importance in the ancestors of existing Vertebrates. 



In all the higher Vertebrates this organ always becomes 



Q 



