I have never had any chance of examining the growth _post mortem_, but the portions which I have removed _en masse_ by operation have enabled me to make some investigations. The periosteum strips off readily, and under this is a thin shell of compact bone, which appears somewhat ridged on the side towards the periosteum. The rest of the tumor consists of cancellous bone. The whole swelling cuts readily with bone forceps and consists of quite soft bone. On making microscopical preparations there were signs of ossification in membrane proceeding under the periosteum, and the rest appeared like ordinary wide-meshed cancellous bone. The whole process appeared to be that of a slow “osteoplastic periostitis.” ÆTIOLOGY. Two views on the ætiology of this disease have been brought forward up to the present time, as far as I know—viz., that the swellings were of a racial character and that the process was started by the larva of some insect. With regard to the first I have only to mention that the disease is found in Ashantis, Grunshis, Fantees, Abantas, the Ga people, etc., races quite different from one another, to show that this cannot be entertained. As to the second, I have never met with evidence which would support the idea that the disease was started by a larva. On the other hand there is always the history of yaws and of the tumor starting during the attack of yaws—_i. e._, during the period of eruption or soon after. Then, again, the patients complain of pain in the nose with, in some cases, distinct history of a sore and sometimes discharge preceding the swelling. This might be due to some irritation or ulceration of the nasal mucous membrane by the yaws. I have never had the opportunity of examining any person at this stage of the disease, but in the more developed cases I have examined the nose for marks or signs of old ulceration, but have not found them. If, however, the nasal process of the superior maxilla be examined a few foramina are to be seen, and these are often joined together by a small groove indicating the position of a bygone suture. The foramina are for small bloodvessels, which are said to communicate with those of the mucosa of the nose. The site of these foramina is the situation where henpuye starts, and I venture to bring forward the theory that the causation of this peculiar disease is due to an osteoplastic periostitis brought about by the absorption of the poison of yaws from the nasal mucous membrane through the small vessels (or lymphatics) keeping open the foramina which indicate the suture above mentioned. THE GEOGRAPHICAL DISTRIBUTION.