406 READINGS IN BIOLOGICAL SCIENCE 



purely a response to physical and chemical stimuli, modified by certain 

 substances secreted by the body and known as "hormones" from the Greek 

 verb honmo, to excite. What are these substances, how are they formed and 

 what role do they play in animal physiology? 



The recognition of the value of certain organs in curing disease goes 

 back to the days of Hippocrates, the "father of medicine," and since his 

 time many such remedies have been proposed. Thus the liver of the pigeon 

 or the wolf were used in cases of diseases of the liver, the rabbit's brain 

 was given for tremors and the lung of the fox for difficulty in breathing. 

 The testicles of the donkey or stag were recommended by Pliny for the 

 renovation of the debauchee. It is now known that one kind of diabetes, 

 which is marked by the presence of sugar in the urine, is not a kidney dis- 

 order, but is due to improper action of the pancreas, as a result of which 

 a specific secretion, passed by the latter into the blood stream and function- 

 ing in sugar metabolism, is absent or reduced in amount. 



Imperfect development of the thyroid gland leads to the condition known 

 as cretinism. Feeding the extract of the thyroid gland of the sheep, or the 

 gland itself, either raw or cooked, result in great increase in growth and 

 development of both mind and body in such cases. Attached to the lower, 

 central part of the brain is a small gland, the pituitary body, which some 

 enthusiastic theorists have fancied to be the seat of the soul. If this gland 

 is partly removed from a young puppy it ceases to grow except for the 

 accumulation of fat. It keeps its puppy hair and milk teeth, while the de- 

 velopment of the genital organs and of the intelligence is much retarded. 



One of the most striking examples of the role of hormones or internal 

 secretions is the action of the sex glands in controlling both body form and 

 mental activity. The physical and mental changes occurring in both boys 

 and girls at the time of puberty are too well known to require even passing 

 mention here, while the dependence on the proper functioning of the sex 

 glands of the secondary sexual characters, such as the horns of the deer, 

 the comb and feathering of the cock, the size of the stallion, is equally 

 f amihar to everyone. Horses and cattle are castrated to render them docile 

 and serviceable as draft animals, and the cock is castrated in order that 

 he may take on more flesh and become a welcome member of our dinner 

 parties. 



Of all the features characteristic of living matter, none is more so than 

 reproduction. Attempts have, it is true, been made to compare the growth 

 of many crystals of salt in a concentrating solution with this miracle of 

 life, but such attempts sound like a mere play upon words. There is noth- 

 ing in the inorganic world in any way comparable to this wonderful 

 phenomenon. Here then, if anywhere in the world of life, we should find 

 evidence of some force higher than the physical forces, did any such exist. 

 But what do we find? The method of reproduction (bi-sexual or partheno- 

 genetic) can be altered by external means; furthermore in Hydra it can 



