3 



32 GENERAL ZOOLOGY 



which were once believed to show the presence of some 

 vital quality are now understood and may be controlled 

 at the will of the experimenter. A turtle's heart may be 

 taken from its body and placed in saline solution where it 

 will continue to beat rhythmically for two or three days. 

 Furthermore, its rate of beat may be regulated. If a cer- 

 tain mineral salt is added to the solution in which it lies, 

 it beats faster; if another is added, it slows down and even 

 ceases to pulsate, but will begin again if the solution is 

 restored to its original condition. 



Another stroke for mechanism came with the knowledge 

 t of the control of " vital" activities through enzymes and 

 ^4jormones. For example, there are in every man a pair 

 ? of glands, the thyroids, on either side of the neck which 

 \ are very necessary to "vital" activity. They give off a 

 substance, thyroidin, which enters the blood stream and is 

 constantly necessary for the proper nourishment of the 

 body. Goitre is the name given to a diseased, swollen 

 condition which may interfere with the proper function- 

 ing of the thyroids. It was in trying to find a cure for 

 this trouble that some interesting facts were discovered. 

 It was found that, if diseased glands were removed, the 

 body would waste away and the patient finally died. But 

 if, after taking out the glands, thyroidin from a healthy 

 sheep was injected hypodermically, there was no dis- 

 turbance. Furthermore the same result could be obtained 

 by grafting a fresh thyroid beneath the skin. Thyroidin 

 is, then, necessary in the body of man, but it makes little 

 difference how it is supplied. The machine needs a certain 

 amount of the substance to run properly but the thyroid 

 gland is not necessary any more than an oil cup is neces- 

 sary for an engine if oil is supplied in some other way. 

 Many other substances control or facilitate various activi- 

 ties which were once believed to be dominated by " vital 

 spirits." 



Arguments might be multiplied which tend to show that 

 living animals are machines, but let us look at the other 



