RESPONSIVE LIFE OF ORGANISMS 



443 



Cell still acts upon its neighboring cell directly; and it also 

 acts on all the cells of the body by contributing certain of 

 its products to the fluids that bathe the whole interior. 

 Many organs are known to produce internal secretions, 

 which are circulated about with the blood, and which pro- 

 foundly affect the well-being of other organs. Thus, the 

 small intestine in vertebrates, when stimulated by contact of 

 the acidulated food entering it from the stomach, produces 

 a substance (known as secretin), which, when carried by 

 the blood to the pancreas, incites that organ to secrete and 

 discharge its own digestive fluid (pancreatic juice). And 

 certain organs, like the thyroid gland (which in us envelops 

 the base of the trachea on the ventral side, and which is the 

 seat of the disease known as goiter), have been found to be 

 of great importance by reason of the internal secretions 

 (hormones') which they contribute to the body fluids. If 

 for example, the thyroid gland be removed, its loss causes 

 both internal and external derangement. The skin thick- 

 ens and becomes wrinkled, the hair falls out, the nervous 

 balance is disturbed, and finally, death results. But if the 

 extract from the thyroid gland of another animal of the 

 same species be injected periodically underneath the skin, 

 these results will not follow the removal of the gland. 

 Similarly, the sex organs appear to produce hormones that 

 condition the development of secondary sexual characters. 

 For when the spermary of a young cockerel is removed, comb 

 and spurs and the other external signs of his sex are checked 

 in their development ; but they can be made to develop by 

 merely grafting into his body a piece of the spermary of 

 another cockerel. So, it appears probable that to this fluid 

 which bathes all the tissues of the body and conditions its 

 metabolism, every cell may contribute something, and the 

 general condition of the body may be largely determined by 

 the totality of this contribution. Internal secretions or 



