Chemical Messengers 



H 



As we have seen, a large community of cells must have controlling 

 mechanisms, not only while the organism is growing but also to 

 secure its harmonious functioning. The different kinds of cells must 

 carry out their functions at the right time. A highly developed animal 

 has of course very delicate mechanisms like nerves which help to 

 control its actions, but it is probable that the earliest methods of 

 control were chemical ones, such as are still found in simple organisms 

 which have no nervous system. Although the nervous system is used 

 for types of control in which speed is essential, nevertheless many of 

 the chemical controls have been retained and are still made use of 

 for many purposes, even in highly developed animals. 



It has been known for many years that animals possess a variety 

 of glands. Some have ducts leading into various organs, other are 

 called ductless glands, because they release their secretions directly 

 into the blood vessels. The importance of these glands began to be 

 appreciated at the end of the nineteenth century, when it was found 

 that the loss of one or more of them had serious effects on the 

 behaviour of the body. Experiments were made on the effects of the 

 removal of the glands of animals and it was found that the symptoms 

 produced could be relieved by injecting extracts obtained from the 

 glands of other animals. It was inferred from this that the glands 

 produced chemical agents, which were called 'hormones' (chemical 

 messengers) many of which have since been isolated. They differ 

 greatly in chemical complexity. Some are quite simple substances, 

 others are proteins. The study of hormones has become one of the 

 largest and also one of the most successful branches of biochemistry. 

 I shall not attempt to give an exhaustive description of these, but a 

 few examples will illustrate their importance. 



As long ago as 1884 it was recognized by Horsley that the removal 

 or atrophy of the thyroid glands, a pair of small glands in the neck, 

 produced cretinism, a kind of atrophy of the body and also of the 



