One of the elements of tobacco, nicotine (which is itself composed of several bases), passes into the blood, and is conveyed in it to the brain. As this is one of the most active poisons of which we have any knowledge, and, as such, acts directly on the brain, rapidly inducing the conditions mentioned, and overpowering the functions of the nervous system, it will readily be comprehended how profound its influence must be upon the nervous filaments and brain-cells. This influence is one of paralysis--a deadening of the functional activity of certain portions of the brain, and, if continued long enough, is radiated through the nerves to the heart and other organs. If, however, a small quantity only has been absorbed into the circulation, the effect upon the brain is less permanent than would be expected from its profound influence. It soon becomes eliminated from the system through the agency of the lungs, the skin, and the kidneys,[20] and the nervous system resumes its normal activity. Subsequent experiences after the first few are widely different. The brain becomes _tolerant_ of the poison, so far as relates to its more manifest and paralyzing effect, and in the process of time the smoker realizes very little, if any, other than a sense of soothing, and, therefore, grateful influence upon the system. The brain even comes, in the process of time and experience, to crave its influence. A condition, abnormal as to experiences and cravings, has resulted, which will demand the frequent and continued repetition of those pleasing sensations which result from its use. When this influence passes off, there comes again into operation a more acute activity of nerve function, and a consequent sensation bordering upon or akin to pain as the brain begins to act under the freedom and force of a vitality not blunted or partially paralyzed by the presence of the poison. To relieve this there must be a repetition of its use, and this process continues until there becomes developed a true _diathesis_ of the nervous system, as in the case of alcohol and opium, though one not, in general, so difficult to overcome. Moreover, the influence of tobacco upon the brain is essentially different from that of alcohol. From the effect of the latter there results a vast increase of functional activity of the heart, and, in the primary stage of its influence, of that portion of the brain which is concerned in mentality, while no such increase of functional activity of any portion of the nervous system results from the physiological effects of tobacco. On the contrary, these are of a paralyzing nature while they continue. The field of consciousness is less broad, the imagination and reason less keen and active.