THE CHECK AND DRIVE SYSTEM
Another instance of the complexity of these inter-glandular reactions is furnished by the thyroid and the adrenals. The thyroid and the adrenals are mutually stimulating—when the thyroid oversecretes, the adrenal dittos, and vice versa. Yet they have directly opposed effects upon the economy—because they act upon antagonistic portions of the involuntary or vegetative nervous system, the system which is independent of the will. Before proceeding further, it is worth while sketching this division of the nervous system.
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.
Normally, the check and drive systems are properly balanced. But under stress and strain the balance is upset. Indeed, the Kinetic Drive may be defined as a mechanism contrived in the course of evolution as the normal, healthy mode for meeting stress and strain. The Kinetic chain of organs, brain, adrenals, liver, thyroid and muscles, began working together in desperate situations for their possessor ages ago. Successful in helping him to survive, they have survived as a functional unit.
It was probably evolved in the Post-Tertiary Era, about twenty million years ago, when the coming of the carnivores introduced direct body-to-body conflicts, and their concomitants, a quick and versatile nervous system. During the Tertiary epoch the earth basked in the heat of a tropical sun nearly everywhere on its surface. The luxuriant vegetation of the torrid zone flourished and swarmed, for the temperature all over was what it is today at the equator. Gigantic vegetarians were the animals, creatures like the dinosaurs, enormous, gargoylean monsters, of an incredible size and strength, but clumsy and grotesque, with small brains and little intelligence. For what need was there for brain and intelligence when food lay about so abundantly at hand for them to gorge themselves. As there was no competition for food, there were no enemies.
Then as the earth evolved and grew cooler, vegetation failed, the ancestors of the present carnivora appeared, the fathers of the wolf and tiger, light, lithe and pugnacious, with senses acute and ferocious weapons of attack, who set out to destroy everybody. They destroyed pretty nearly all of the huge leaf-eating species, and only the more plastic and smaller ones, who were more keen-sensed and swift-footed (of whom the deer and antelope, horse and ox are the descendants), escaped. The smallest either took to the air to become the bat, or, like the forerunners of the squirrel and ape, took to the trees.
It was the coming of the carnivores, therefore, that accelerated the development of brain matter, and started the process which created man. But in the millions and millions of years of conflicts, instincts grew into being that sank deep into bone and marrow. The most fundamental reflexes, those immediate responses to irritation or danger, were laid down, and among them the drive and check system. When the animal had decided to fight its enemy or was forced to fight, or determined to prey, then was the time for the drive system to do its utmost to speed up everything that would help in the fight, while the check system came into play to hinder whatever would interfere or burden in the fray. First the drive mechanism must have been hit upon, and then the value of the check devices must have been found in fear and flight, and especially in hiding and simulation of death, when even breathing had to be inhibited. Until finally there developed, for everyday use, a complete check and drive nerve machinery for every organ, to be used according to the exigencies of the moment, with the thyroid as the primary stimulant and controller of the drive system and the adrenal as the primary dictator over the check system.