If we run one line between the serous and another between the mucous membrane and the central line, as in the diagram, other strata with intervening spaces will be indicated. _a_ would represent the centre stratum of the wall; _b_, the space immediately outside of that; _c_, a stratum still farther out; _e_, the subserous; and _d_, a deeper one. When we look at the inner layers of fibres, we find _f_ situated immediately beneath the mucous membrane; _g_, farther out; and _h_, next the median line. The nucleus of a tumor may be first manifested in any of the strata or spaces marked by these lines, and its position with reference to the central line will, to a great extent, govern the direction it takes during development. A tumor the nucleus of which is situated in line _a_ will, as it develops, press the muscular fibres equally in every direction, and when large, the prominence caused by pressure of the tumor would be equal in the uterine cavity and on the peritoneal surface. In marked contrast to this, when the nucleus is at _f_ the growing tumor presses the mucous membrane before it until it becomes pendulous, and then the name of polypus is given to it; or if the origin is at _e_, the serous membrane is pressed before it, and the tumor is called subserous. When the nucleus is at _d_, the tumor elevates the serous membrane and becomes a prominent hemispherical protuberance. It is also called a subserous tumor, although situated some distance from the membrane. When a tumor takes its origin at _g_ the mucous membrane is crowded before it, and a marked prominence into the cavity of the uterus is observed. This is the submucous tumor. These illustrations are intended to call the attention of the student to the fact that practically these tumors spring {250} from any one or all the fibrous strata of the uterus instead of only the central, submucous, and subserous layers, and that it is profitable, on account of the difference in their effects upon the shape and functions of the uterus, to study them in this aspect of their growth. ETIOLOGY.--While we know many of the conditions under which fibrous tumors exist, we have really very little, if any, definite and reliable information as to their causes, either remote or proximate. We know that they occur much more frequently near the time when the uterus begins to undergo senile degeneration, although they do originate in earlier years. They very seldom, if ever, are observed in the foetus or child, nor is it common for them to commence growing after the menopause. Women belonging to the African race are the most frequent subjects of these tumors.