1186 



PHYSIOLOGY 



work may show some connection between the cortex and the destruction of 

 pigment in the body. At present it is only by a process of exclusion that 

 we may guess at a causal relationship between the destruction of the cortex 

 and the bronzing which occurs in Addison's disease. 



There seems little doubt that the rapidly fatal effects of extirpation of 

 both suprarenals is to be ascribed rather to the removal of the cortex than of 

 the medulla. The functions of the latter can be more or less effectively 

 maintained by the other chromafnne tissues found at the back of the 

 abdomen. In the few cases where animals have survived double extirpation 

 small masses of accessory cortical substance have been found embedded in 

 the kidney or elsewhere in the neighbourhood of the suprarenals. 



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. Sur- 

 rounded by a capsule of connective tissue, it is made up of an aggregation of 



vesicles varying in size from 15 to 

 150yu. The vesicles are lined by 

 a single layer of cubical epithelial 

 cells, and are filled with a trans- 

 lucent material known as colloid 

 (Fig. 546). Of the cells, some 

 present granules and resemble the 

 cells of a secreting gland, while 

 others contain masses of colloid, 

 or have undergone colloidal de- 

 generation. Between the vesicles 

 may be seen, here and there, solid 

 masses of cells which by some 



FIG. 546. Section of thyroid gland of dog. 

 ( SWALE VINCENT.) 



observers are regarded as destined 

 to replace vesicles the epithelium 

 of which has undergone complete 

 degeneration. The colloid material can be traced between 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 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 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 

 bslow 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 



