Structure of Thyroid 



13 



solid substance by fixative reagents. The intervesicular substance is 

 areolar tissue, containing in parts many small cells. Some of these are 



Fig. 3.— Section of thyroid (cat). Magnified 300 diameters. 



lymphocytes, whilst others are not unlike those of the parathyroids, 

 although the identity has not been established. There are also very 

 numerous blood-vessels, the thyroid being one of the most vascular organs 

 in the body ; it receives in proportion to its size 

 more than five times as much blood as the 

 kidneys. The capillaries form a close network 

 around each vesicle, giving a characteristic ap- 

 pearance to an injected specimen (fig. 4) : they 

 come into immediate contact with the vesicular 

 epithelium (fig. 5). There are numerous lym- 

 phatics within the organ, and " colloid " like 

 that of the vesicles is occasionally seen within 

 them. The gland receives nerves from the 

 sympathetic and from the superior and inferior 

 laryngeal : they are distributed both to the blood- 

 vessels and to the secreting epithelium. 



The vesicles of the thyroid do not always 

 present the appearance above described. Sometimes they are smaller 

 or irregular, with projections into their cavities, and the lining epi- 

 thelium cells are pronouncedly columnar. In these circumstances there 

 is usually little or none of the typical colloid material in the vesicles — 

 although the appearance of the cells is suggestive of secretory activity. 

 It may be inferred that the secreted material finds in these cases a 



Fig. 4. — Thyroid of dog 

 injected, low power. 



