64 



THE HUMAN MECHANISM 



as a whole ; and it is probably true that not only in such 

 abnormal conditions but also in the daily conduct of life the 

 fatigue of moderate muscular activity contributes its share 

 toward inducing healthful and refreshing slumber. 



Thus far we have considered the chemical activities of 

 each organ as contributing to the work of the organ in 

 which they occur and, because of the accumulation of waste 



products, as the occasional 

 cause of undue interference 

 with efficient activity, both in 

 the working organ and else- 

 where. And yet the familiar 

 case which we have just cited 

 suggests another view of the 

 matter. The products of the 

 chemical activity of one organ 

 may be of service to other 

 organs, and so to the body 

 as a whole; and while their 

 too rapid accumulation in the 

 FIG. 36. Cross section of the thyroid blood ma J be undesirable, 



gland 



their presence in moderate 



The cells secrete into the closed sacs, amounts may be beneficial 

 ^tUS7i and may contribute to the 

 the cells into the lymph spaces of the normal environment of the 



connective tissue ^^ f .-, , * 



cells of the body. 



2. The thyroid gland. This view of the case is strikingly 

 emphasized in the physiology of the thyroid gland a small 

 organ in the neck, the two chief lobes of which lie alongside 

 the trachea. For a long time its use was not understood, 

 and at times it was even supposed that it plays no important 

 part in the life of the body as a whole. It has been found 

 by experiment, however, that removal of the thyroid is fol- 

 lowed by a disease in all respects similar to one which had 

 long been observed in human beings, especially in children; 



