Under ordinary circumstances, the impression made upon the nervous tissue, and the motor impulse which is generated in consequence of it, are sufficient and no more for effect- ing the purpose required in sustaining the economy. But in certain diseased conditions, the susceptibility to impressions, and the power of generating motor impulses, are greatly increased ; of this we have examples in hydrophobia, tetanu?, and in some forms of epilepsy, chorea, and hysteria. It is a singular fact, that withdrawal of the influence of the will seems to have the effect of exalting the excito-motor functions. Thus, in paralysis of one of the lower extremities, tickling the sole of the foot upon the side affected, will often produce more violent muscular contractions than upon the opposite or healthy side. The condition now under consideration is sometimes produced by the introduction of certain poisons, strychnia, &c.
CHAPTER II.
DERANGEMENTS OF THE BLOOD.
All nutritive processes and vital actions depend upon a healthy or normal condition of the blood, any considerable deviation from which is consequently an important element in the production of disease. Such deviations are of frequent occurrence. Not only may the blood be changed by a relative disproportion in the amount of its various constituents, but it is liable to be rendered impure by the reception* of poisons from without, or by the absorption or retention of matters from- within. Moreover, anything which interferes with nutrition is a source of impurity. For it is to be observed, that those constituents of the blood which are separated from it, and applied to the nourishment of the tissues, would, if they were retained, become foreign or poisonous agents.
Unhealthy conditions of the blood sometimes find expression in a general derangement or commotion of the system, as is observed in certain fevers. At other times the morbid effect is localised, disease being set up in a part or organ. Of this,
