SEC. 6. ON SOME STRUCTURES AND PROCESSES OF 

 OBSCURE NATURE. 



494. The Thyroid Body. Certain structures of obscure 

 nature, but probably connected in some way or other with 

 some of the metabolic processes in the body, are often spoken 

 of under the undesirable name of ' ductless glands.' Such are 

 the thyroid body or gland, the pituitary body, the thymus, and the 

 suprarenal capsules. These differ from each other so essentially, 

 that the only plea which can be urged in favour of considering 

 them together is convenience and our limited knowledge of their 

 respective functions. 



The thyroid body is the one of the group most deserving to be 

 called a gland, since it, like the lungs, arises as a two-lobed 

 diverticulum from the ventral surface of the anterior part of 

 the alimentary canal, and at first, like the lungs also, behaves 

 as if it were about to become a double racemose gland. The 

 connection with the throat however, which should have become 

 a duct, is soon obliterated, and the two lobes, united with 

 each other by an isthmus across the trachea, lose all traces 

 of any branching ducts within them and become transformed 

 into masses of isolated, ductless alveoli bound together with 

 connective tissue. 



Hence, when a section is taken through a hardened and 

 prepared lobe of an adult thyroid, what is seen is a limiting capsule 

 of connective tissue sending into the interior numerous septa, 

 which surround and separate from each other round or oval spaces, 

 the sections of the isolated alveoli. These are of variable size, 

 some being visible to the naked eye, and each is lined by a single 

 layer of low columnar or cubical nucleated cells resting on a base- 

 ment membrane, leaving a large cavity, which is filled with 

 material varying in consistency from a mere glairy fluid to a 

 clear, almost solid substance. In the septa among the alveoli 

 are groups of cells not surrounding any cavities ; some of these 

 groups appear to be developing alveoli. 



The septa of connective tissue, fairly rich in elastic elements 

 but remarkably free from adipose tissue, contain numerous blood 



