1238 



PHYSIOLOGY 



the kidney or elsewhere in the neighbourhood of the suprarenals. Hyper- 

 trophy or a tumour of the suprarenal bodies, involving the cortex, has been 

 found associated in children with premature sexual maturity. 



FIG. 559. Section of thyroid gland of dog. 

 (SWALE VINCENT.) 



THE THYROID GLAND AND THE PARATHYROIDS 



The thyroid gland consists of two oval bodies lying on either side of the trachea, 

 joined in many animals across the trachea by an 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. 559), 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 degeneration. 

 The colloid matter can be traced be- 

 tween the cells into the lymphatics 

 lying between the vesicles. Since the 



gland possesses no duct, it is supposed that the cells furnish an internal secretion, 

 which makes its way into the blood along the lymphatic efTerents 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 tlie fore- 

 gut, but the connection with the gut disappears 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 as myxoedema with atrophy of the thyroid . 

 A patient affected with myxcedema presents a gradually increasing blunting 

 of his or her mental activities; speech is slow, cerebration delayed. With 

 this nervous defect are associated changes in the connective tissues, the 

 subcutaneous connective tissue becoming thickened, so that the face and 

 hands appear swollen and puffy, looking at first sight as if oedema were 

 present. The swelling is however due to newly formed connective tissue, 



