THE RESPIRATORY ORGANS. 257 



principally of medullated fibres ; fibrillae are traceable into the sub- 

 pleural tissue, but their exact mode of ending is uncertain. 



THE THYROID BODY. 



In view of its topographical relations, as well as a matter of con- 

 venience, it is usual to consider this organ in connection with the 

 respiratory tract, although such association is only incidental and 

 without foundation or morphological significance, unless its descent 

 in common with the respiratory organs as an outgrowth from the 

 pharyngeal entoderm be regarded in such light. 



The thyroid body is a compound tubular gland whose excre- 

 tory canal, the thyro-glossal duct, in the early stages of the organ, 

 connects the tubules with the mucous surface, where its opening 

 corresponds to the foramen caecum, situated on the dorsum about 

 an inch from the base of the tongue. After a short existence, long 

 before the gland attains its full development, the thyro-glossal duct 



FIG. 292. 



Section of thyroid body of child : a, acini distended with colloid secretion, cut in 

 various directions ; b, interlobular connective tissue. 



undergoes atrophy and more or less complete obliteration ; the acini, 

 consequently, become isolated closed cavities, while the organ is 

 often classed as a ductless gland. 



The fully-developed adult thyroid gland consists of numerous 

 tubular acini, 40-110 p. in diameter, united by intertubular areolar 

 tissue into lobules ; these, in turn, are joined into lobes by still 

 larger masses of connective tissue, which form on the outside of the 

 organ a general external fibrous envelope. 



The acini are completely closed, and lined with a single layer of 

 cuboidal or low columnar epithelium, whose component cells rest 

 upon a distinct basement-membrane. The enclosed cavities differ 



17 



