In the construction of a motor car from the point of view of absolute control of it at every moment, the first thought of the mechanic is an adequate _brake_ and an efficient _regulator_ of speed, instruments antagonistic, but necessary to work simultaneously or alternately. The involuntary or vegetative nervous system is built upon the same principle. It supplies every organ in the body beyond the control of the will (that is to say, the brain) with two sets of filaments which have opposing functions. One group of filaments in general increases or activates the function of the organ to which it is distributed. The other group of filaments, when tingling, inhibits or prohibits that function. They are like the two buttons on the wall which regulate the supply of electricity to incandescent bulbs, one switching on the current, the other switching it off. It has been agreed to call the stimulative or activating portion the autonomic or drive system. To its antagonist has been left the older name of the sympathetic or check system. It is because they do not both act upon these two components of the vegetative nervous system, but only upon one, that the thyroid and adrenal though in themselves complementary, come to exert opposite effects. For the internal secretion of the thyroid has a selective affinity for the autonomic or activating system, while that of the adrenals has a selective affinity for the sympathetic or inhibiting system. In the stomach, for instance, extracts of the adrenal glands have been proved to intensify the function of the sympathetic or check system in different degrees, so that there is a lessening of the amount and acidity of the gastric fluid. On the other hand, thyroid extracts will intensify the action of the autonomic or drive system, so that the amount and acidity of the digestive juice is increased. The stomach cell may, therefore, be regarded as a test-reagent for the different internal secretions, as they affect the check and drive systems. These constitute an automatic device for regulating the activities of every organ. Three factors enter into the mechanism. One is the amount of the circulating internal secretions. Another is the organic and functional integrity of the nerve filaments comprising the check and drive systems. The third consists of the number and vitality and limitations of the terminal receiving cells acted upon by the nerve filaments, which in their turn have been acted upon by the internal secretions. Upon every organ, including the mind, through the brain, a stimulus from without or within will act according to its ability to influence one or others of these factors.