RESPIRATORY ORGANS. 



33 



tion, these also separate from the parent epithelium, and become 

 joined to the anterior portion in mammals, but retain their 

 distinctness in the lower forms. The thyroid takes first the 

 shape of numerous cylindrical cords with internal lumen. The 

 cords branch and anastomose, and blood-vessels 

 and connective tissue enter the network thus 

 formed. 



By most students the unpaired portion of 

 the thyroid gland is considered as a derivative 

 of the hypobranchial groove of the tunicates 

 and Amphioxns, while the paired portion is 

 probably to be regarded as derived from an ad- 

 ditional pair of gill slits which never break 

 through to the exterior. The function of the 

 thyroid is apparently to form some compound 

 of iodine necessary to keep the system in good 

 condition. 



Thymus Gland. Closely connected with 

 the gill clefts in development are a pair of 

 structures of problematical functions, the 

 thymus glands. Each arises from the epithelial 

 tissue on the dorsal margin of one or more 

 gill slits. Later this tissue becomes invaded 

 by leucocytes, while ingrowths of connective 

 tissue divide it up into small lobules. In the 

 fishes the glands remain near these points of 

 origin just above the gill slits ; in the amphibia 

 they occur above and behind the angle of the 

 jaw. In the reptiles the glands occur in the 

 neck, sometimes far anterior, sometimes, as in 

 the snakes, just in front of the heart. In the 

 birds these glands extend nearly the whole 

 length of the neck, while in the mammals they pass backwards 

 into the thoracic region immediately behind the sternum, and 

 ventral to the heart. 1 In mammals the thymus glands undergo 

 more or less complete degeneration with age ; in man they 

 reach their highest development in the second year. 



1 The thymus glands of calves are the 'throat sweetbreads' of the market. 



FIG. 36. Rela- 

 tions of thyroid 

 ( 77v) and thymus 

 ( TM} glands in an 

 infant, after Wie- 

 dersheim. , epi- 

 glottis; //, hyoid 

 bone; T, trachea; 

 TV, thyroid carti- 

 lage. 



