ENDOCRINE GLANDS 25 



to undergo atrophy, but they seem to be able to differen- 

 tiate in response to a variety of stimuli, and presumably their 

 existence is related to the development of adenomata of the organ. 

 In the lining cells of the acini the Golgi system usually lies 

 between the nucleus and the lumen, but in a small percentage of 

 the cells this structure lies on the other side of the nucleus. 



PARATHYROIDS 



These small organs lying so close to the thyroid exert a great 

 influence on many phases of growth, and do not appear to have 

 any functional connection w4th their much larger neighbour. 



There are three types of cells in these glands : clear chief cells, 

 which appear to be the most important elements, and, in man, 

 compose the w^hole organ up to about the tenth year of life. 

 There is a close resemblance between these elements and the lay 

 idea of a cell because the protoplasm is so pale as to be almost 

 invisible, whilst the cell boundary is quite sharply defined. Large 

 eosinophilic cells with small deeply staining nuclei and very 

 granular cytoplasm. 



The third group are probably only manifestations of the chief 

 cells. 



Nothing is known as to the diversity of function that may be 

 inferred from the histological characters. Even the situation of 

 the Golgi apparatus does not give much indication as to the 

 direction in which secretion normally occurs, because this structure 

 is so variable in its intra-cellular position. The follicles containing 

 colloid that develop in the parathyroids in later life misled many 

 of the earlier workers into the idea that these organs were nothing 

 more than phases of thyroid activity, but it is now recognised that 

 almost any secretion, when seen in fixed tissues, is morphologically 

 identical with thvroid colloid. 



THYMUS 



The thymus is one of the most interesting and puzzling structures 

 in the animal body, and we must be excused if much of what we 

 describe is not strictly new, but there is so much confusion about 



