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 cap- 

 sules. 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 ignorance 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 in fresh specimens 

 is filled with a glairy fluid. The cells present no special characters. 



The septa of connective tissue, fairly rich in elastic elements 

 but remarkably free from adipose tissue, contain numerous blood 

 vessels derived from the superior and inferior thyroid arteries, the 

 branches of which, relatively large and frequently anastomosing, 

 end for the most part in capillary networks round the alveoli ; 

 from these capillaries and those of the septa the blood is gathered 



