???? Every motor nerve must be balanced by an inhibitory nerve. The one furnishes the driving force, the other applies the brake. For instance, the heart muscle is supplied with motor force through the spinal nerves from the upper dorsal region, while the pneumogastric [vagus] nerve retards the action of the heart and in that way acts as a brake. ???? In this way, by means of the inhibitory nervous system and of the accumulating fatigue products in the body, Nature forces the organism to rest and recuperate when the available supply of vital force runs low. The lower the level of vital force, the more powerful will become the inhibitory influences.
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???? Another brake is supplied by the waste products of metabolism?in the system, the uric acid, carbonic acid, oxalic acid, etc., and the many forms of xanthines, alkaloids, and ptomaines. As these accumulate in the organism during the hours of wakeful activity, they gradually clog the capillary circulation, benumb brain and nerves, and thus produce a feeling of exhaustion and tiredness and a craving for rest and sleep.
