In the first place certain normal bursæ in important situations are frequently so much enlarged by the constant irritation of pressure and friction that they become considerably enlarged. This enlargement may go on to definite pathological changes and thus come under the care of surgeons. They are Prepatellar bursæ--“housemaid’s knee.” Olecranon bursæ--“student’s elbow” and “miner’s elbow.” Tuber ischii bursæ--“weaver’s bottom.” These may be called “occupation-bursæ” and may be classed with three other well-known adventitious bursæ which are formed on the shoulder in “deal runners,” on the scalp in “fish porters” and in the back of the neck in Covent Garden porters, known as a “hummy.” Entirely new bursæ are formed also over the cuboid bone in talipes equino-varus, over the internal condyles of the femur in bad cases of knock-knee from friction of one joint against the other, over the prominent vertebrae in a humpback. A structure closely resembling a bursa and arising from similar causes to those producing adventitious bursæ is found in unreduced dislocations or ununited fractures. A small example of an adventitious bursa came under my notice. A woman, E. L., aged 49, had remarkable enlargement of the metatarso-phalangeal joint of her great toe of the left foot, and over this joint was formed a well-marked bursa on the dorsal surface. The right foot showed a much less prominent joint and only a very slight development of the corresponding bursa. This instance of a bursa-like structure being produced in unreduced dislocations and ununited fractures suggests the conception which I here propose, but do not attempt to verify that _all joints in all animal forms from the lowest up to man have been evolved in a manner to which this pathological experiment may give a clue_.