1326 PHYSIOLOGY 



isthmus. Surrounded by a capsule of connective tissue, it is made up 

 of an aggregation of vesicles varying in size from 15 to 150 /x. The 

 vesicles are lined by a single layer of cubical epithelial cells, and are 

 filled with a translucent material known as colloid (Fig. 544). Of the 

 cells, some present granules and resemble the cells of a secreting gland, 

 while others contain masses of colloid, or have undergone colloidal 

 degeneration. Between the vesicles may be seen, here and there, solid 

 masses of cells which by some observers are regarded as destined to 

 replace vesicles the epithelium of which has undergone complete degene- 

 ration. The colloid material can be traced between the cells into the 

 lymphatics lying between the vesicles. Since the gland possesses no duct 



FIG. 544. Section of thyroid gland of dog. (SWALE VINCENT.) 



it is supposed that the cells furnish an internal secretion, which makes 

 its way into the blood along the lymphatic efferents of the gland. 

 The thyroid is richly supplied with blood by the superior, middle, and 

 inferior thyroid arteries, and is surrounded with a plexus of veins lying 

 immediately under the capsule. In development the thyroid is formed 

 by an outgrowth from the fore-gut, but the connection with the gut dis- 

 appears long before the end of foetal life. In rare cases part of the 

 duct may persist, and, becoming gradually filled with fluid, give rise 

 to a hyoid cyst which lies below the tongue and may require excision 

 by the surgeon. 



As in the case of the other ductless glands, clinical observations 

 have contributed materially to our knowledge of the functions of the 

 thyroid. Although the gland had been extirpated in animals by 

 Astley Cooper and by Schiff, the attention of physiologists and medical 

 men was especially directed to the importance of this organ by the 

 observations of surgeons, especially Kocher, on the untoward and even 

 fatal effects following its complete removal in man in operations for 

 extirpation of goitre. In this country attention had already been 

 called to the connection of a disturbed condition of metabolism known 



