Overgrowth is the meaning of this word ; increase in size without essential change in the nature of a part. An organ may enlarge very much, with a great change in its character ; for example, a tumor of the breast, or a dropsy of the head. Again, an organ may be stretched or dilated without even an increase of its substance.

The heart exemplifies two of these changes in different instances. If one of its valves through which the blood passes becomes obstructed from disease, the heart has to labor more than usually to compel the blood to pass by the obstruction. Like other muscles (the heart being really a hollow muscle), this extra labor may have either of two results, according to the conditions present. If the person's constitution be strong, and his blood well nourished, the much-worked heart will grow thicker and more powerful with the exercise. This is hypertrophy. But, if the contrary be the case, with a feeble system and poor blood, the heart is weakened by its excess of labor, and it stretches or becomes thin (attenuated) and dilated.

The thickening of the skin of a working-man's hands shows an increased growth from habitual rough usage. A corn is a hypertrophy, and so is a wart; both involving almost entirely the outer skin or cuticle. Wens and pimples show a greater change of substance with enlargement.