THE VASCULAR GLANDS. 



339 



respiration and the temperature of the body in the reduced state to 

 which they fall during that time. It has been shown also to be a source 

 of the red blood-corpuscles, at any rate in early life. 



The Thyroid. 



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

 one on each side of the trachea extending upwards to the thyroid carti- 

 lage, covering its inferior cornu and part of its body; these lobes are 

 connected across the middle line by a middle lobe or isthmus. The thy- 

 roid is covered by the muscles of the neck. It is highly vascular, and 

 varies in size in different individuals. 



Structure. The gland is encased in a thin transparent layer of dense 

 areolar tissue, free from fat, containing elastic fibres. This capsule 

 sends in strong fibrous trabeculae, which inclose the thyroid vesicles 



FIG. 268. Part of a section of the human Thyroid, a, fibrous capsule; 6. thyroid vesicles filled 

 with, e, colloid substance; c, supporting fibrous tissue; d, short columnar cells lining vesicles; /, 

 arteries; gr, veins filled with blood; ft, lymphatic vessel filled with colloid substance, x (S. K Alcock.) 



which are rounded or oblong irregular 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 a coagulable fluid or transparent 

 colloid material. The colloid substance increases with age, and the cav- 

 ities appear to coalesce. In the interstitial connective tissue is a round- 



