l8 THE ANIMAL ORGANISM 



growth the formation of the various parts of the body needs very 

 strict adjustment. In all such respects the processes of the body 

 are subject to co-ordination. This is effected in animals by the two 

 systems of communication within the body — the blood vessels 

 or other transporting system, and the nervous system. 



Substances secreted into the blood by various organs affect the 

 working of other organs which they reach in the course of the 

 circulation. Some of these substances are not produced ad hoc. 

 Thus the carbon dioxide passed into the blood by active organs 

 as a result of the oxidation going on within them alters the degree 

 of acidity or alkalinity of the blood, and this regulates the 

 quantity and quality of man's blood supply, the acidity causing 

 small local blood vessels to dilate, so that the active organs 

 are flushed with the blood they need, and stimulating the part 

 of the brain which governs respiration, so that rapid breathing 

 oxygenates the blood and removes the excess of carbon dioxide. 

 But the most remarkable instances of regulation of this kind 

 are effected by the secretion in small quantities of very powerful 

 special agents known as hormones. Various organs despatch 

 these, but the most conspicuous examples of their formation are 

 afforded by the ductless glands. About them we shall have more 

 to say later on (see p. 367), and one example of their functioning 

 must suffice here. The adrenal bodies, little glands which lie 

 near the kidneys of backboned animals, are, in moments of anger, 

 fear, or other emotions which forerun violent exertion, caused, 

 by stimuli received through the nerves, to discharge into the 

 blood small quantities of the substance adrenaline. This is carried 

 round in the circulation and tunes up the body for the crisis. 

 It increases the flow of blood in the muscles and brain by quicken- 

 ing the heart beat and constricting the blood vessels of the viscera, 

 augments the supply of fuel for muscular action by causing the 

 liver to pour sugar into the blood, and in other ways prepares the 

 animal for action. The passing of secreta into the blood instead 

 of into tubes (ducts) to be led to their destination is known as 

 internal secretion. The other conducting system, the nervous 

 system, is set into regulative action sometimes by the action of the 

 blood upon the central nervous organ, as in the case of breathing 

 mentioned above ; but more often messages sent in along nerves 

 from organs are translated at the centre into outgoing messages 

 to other organs, whose action they regulate appropriately. By 

 them the contraction of muscles, the secretion of glands, the 



