1. We have described here various forms of rickets and osteomalacia that we have studied ourselves and have come to recognise as comprising definite syndromes. We have included only diseases in which hereditary factors have been proved or might be suspected to play a part.

2. There are more of these syndromes than previously defined.

3. All these hereditary diseases can be treated, usually very satisfactorily, using the same general principles and with only very few simple medicines such as vitamin D and sodium bicarbonate. Careful control and long follow-up, however, may be required to achieve best results and to avoid accidents.

4. A study of the genetics of the diseases is a help in diagnosis and treatment. Broadly speaking the group of diseases which mimic vitamin D deficiency more or less closely tend to be inherited in dominant fashion. The groups with more severe degree of renal-tubule abnormality are inherited as recessives.

5. When more than one member of a family is affected it has been found that in each family each syndrome runs true to type.

1. The range of variation in full extension at the interphalangeal and metacarpo-phalangeal joints of the thumbs of 133 male and 100 female Europeans, and of 31 male Indians and 30 male Africans, has been investigated.

2. There is considerable variation between individuals in the maximum extension of both joints of the right and left thumbs in all groups studied.

3. The distribution for each joint in both thumbs in all groups is fairly symmetrical.

4. There is a high correlation between the right and left thumbs for both joints in all groups

5. The mean angle of extension at the right and left metacarpo-phalangeal joints in all groups is similar. Female Europeans, however, show a significantly greater mean angle than male Europeans.

6. The mean interphalangeal angle of extension in male Europeans is significantly greater than that in female Europeans and the mean in the Indian and African groups is significantly greater than in the male European group.

7. There is slight negative correlation between the metacarpo-phalangeal angle and interphalangeal angle in each thumb in the European groups.

8. Many subjects in all groups can increase extension at the metacarpo-phalangeal joint after flexing the carpo-metacarpal joint. Marked hyperextension (over 40°) is more frequent in the left than in the right thumb, in females than in males, and in male Indians than in male Europeans and Africans.

9. Maximum extension at the interphalangeal joints is not related to the presence of a sesamoid bone in the anterior part of the capsule of the joint.

10. The surfaces of the metacarpo-phalangeal joints vary considerably in shape. Those which are flat form about 10 per cent. of the sample and do not show hyperextension.

11. The factors influencing the amount of extension at the interphalangeal joint is the degree of laxity of the anterior capsule. The problem at the metacarpo-phalangeal joint is more complex; both the capsule and the shape of the joint surfaces play important roles.