198 ANIMAL BIOLOGY 



The almost uncanny potency of hormones in general will be 

 evident from the fact that about 1/1, 000th of a gram of thyroxine 

 is sufficient to induce a 2 per cent increase of the total oxidation 

 of the resting adult human body. The amount of thyroxine re- 

 quired by the body during a whole year is probably about 1\ grams, 

 while the amount in use at any one time is approximately 2/10th 

 of a gram. "But this pinch of material spells all the difference be- 

 tween complete imbecility and normal health" — a fact that 

 should give pause not only to the biologist but also to the sociolo- 

 gist. 



Moreover, as a further indication of the nicety of the reciprocal 

 adjustments within the organism, it may be mentioned that the 

 thyroid gland itself is affected by regulating stimuli reaching it 

 through the nervous system, and also by a hormone derived from 

 the pituitary gland which is another endocrine organ situated in 

 conjunction with the lower surface of the brain. 



Finally, the adrenal glands, one situated in close proximity to 

 each of the kidneys, are endocrine organs of high significance that 

 secrete at least two hormones. One, known as adrenaline, is 

 poured into the blood when the glands are stimulated by the ner- 

 vous system. Adrenaline has its most marked effect when muscu- 

 lar exertion is at a premium. It accelerates heart action, increases 

 blood pressure, reduces muscular fatigue, stimulates the liver to 

 give up its stored glycogen, retards the activities of the alimentary 

 canal, dilates the eyes, etc. It is essentially a chemical whip which 

 makes various organs play their part in the general mobilization. 

 Glimpses of such interrelationships are being gradually afforded as 

 one hormone after another is discovered and studied — chemical 

 coordination is indeed a fact. (Figs. 107, 108, 132.) 



B. Coordination by the Nervous System 



Although hormones are indispensable as a means of regulating 

 many of the processes of the organism, they are entirely inadequate 

 for the instantaneous correlation of diverse parts of an animal and 

 also for the adjustment of the whole animal to its surroundings. 

 This need is supplied in the Metazoa by the nervous system: a 

 complicated arrangement of cellular elements in which irritability 

 and conduction are highly developed. The study of the nervous 

 system constitutes the science of neurology. 



