A. =Physiological Atrophy.=--1. This is always the result of disuse or impaired function from any cause. Its evidences are generally seen in the fatty structures and muscles--_i. e._, in the soft parts. It is true, however, even of the bones, or, of greater interest, even in the brain cells. We see evidences of it also in minute organs; as, for example, in the digestive glands in certain cases where diet is restricted. Again, we see it in the diminution of the size of the heart after hip amputation, less being required of that organ, and also in the entire structure of the rectum after colostomy. 2. Examples of the _developmental type_ are best seen in the natural disappearance of the hypogastric arteries, the ductus arteriosus, the vitelline duct, the Wolffian bodies, and in the various generative ducts (Gärtner’s, etc.) shortly after the birth of the human individual. We sometimes see it also in the prostate after orchidectomy. Equally illustrative is the disappearance of the tail and gills of the tadpole, the eyes of animals living in caverns, and, in a general way, of organs which become useless owing to a different environment. 3. _Senile_ atrophy is seen equally well in the hair follicles, the teeth, the bones, and the sexual organs of elderly people--in fact, in all their tissues, even in the brain. B. =Pathological Atrophy.=--4. Acute atrophy of surrounding tissues is the necessary accompaniment of destruction by suppurative or other disturbances; that is, parts disappear by absorption which have not been interfered with by pyogenic organisms. So complete may atrophy occur under these circumstances as to cause disablement of an organ or part. This kind of senile disappearance is merely an expression of phagocytic activity, although not now a question of bacteria. 5. The same is true of that variety spoken of above as _biological_ or _developmental_, since phagocytes are the active agents in producing the disappearance of the tadpole’s tail. 6. A more slow form of pathological atrophy is seen in the _gradual disappearance_ of tissues in the neighborhood of advancing tumors, enlarging cysts, etc. This is perhaps but another expression of atrophy from continuous pressure. But a still better illustration is the atrophy which comes from immobilization of a part without pressure. This is usually the case when splints or orthopedic apparatus have to be kept in place for some time.