CH. XXV.] THE THYROID 337 



Hassall (fig. 279), which are nests or islands of epithelial cells cut off 

 from the epithelium of the pharynx in process of development. 



The functions of the thymus are very 

 obscure. It has generally been assumed that 

 the lymphoid tissue of which it is composed 

 form colourless corpuscles ; but Stohr asserts 

 that it is not true lymphoid tissue. 



It has been stated that in hibernating 

 animals, in which it undoubtedly persists 

 throughout life, that as each hibernating 

 period approaches the gland enlarges, and its 

 cells become laden with fat. In this case, the 

 store of fat will serve to maintain combustion 

 processes during the winter sleep. ceiis ; 6 corpus 



TH i D ^ ^ j ' ii A. A. j Hassall. (Cadiat.) 



Eemoval of the gland in the frog is stated 



to be followed by muscular weakness, paralysis, and finally death ; 

 but later observations have failed to confirm this result, either in 

 frogs or mammals. Intravenous injection of extracts of thymus 

 lowers arterial blood-pressure and accelerates the heart, but extracts of 

 most organs produce similar effects, especially on the blood-pressure. 



Lately it has been suggested that there is some relationship between 

 the thymus and the generative organs ; and this view is supported by 

 the circumstance that castration retards the atrophy of the thymus, 

 whilst removal of the thymus hastens the growth of the testes. 



The Thyroid. 



The thyroid gland is situated in the neck. It consists of two 

 lobes, one on each side of the trachea; these lobes are connected 

 across the middle line by a middle lobe or isthmus. It is highly 

 vascular, and varies in size in different individuals. 



The gland is encased in a capsule of dense areolar tissue. This 

 sends in strong fibrous trabeculae, which enclose the thyroid vesicles 

 which are rounded or oblong sacs, consisting of a wall of thin 

 hyaline membrane lined by a single layer of short cylindrical or 

 cubical cells. These vesicles are filled with transparent colloid 

 nucleo-protein material. The colloid substance increases with age, 

 and the cavities appear to coalesce. In the interstitial connective- 

 tissue is a round meshed capillary plexus, and a large number of 

 lymphatics. The nerves adhere closely to the vessels. 



In the vesicles there are sometimes, in addition to the yellowish 

 glassy colloid material, epithelium cells, colourless blood-corpuscles, 

 and also coloured corpuscles undergoing disintegration. 



It is difficult to state definitely the function of the thyroid body ; 

 it is one of those organs of great importance in the metabolic round ; 

 and its removal or disease is followed by general disturbances. It no 



Y 



