OSSIFICATION OF THE THIRD TROCHANTER IN MAN. By A. FRANCIS DiXoN, M.B., Chief Demonstrator ofAnatomy, Trinity College, Dublin. SOME time ago I noticed that the femur of a young subject dissected in the School of Anatomy, Trinity College, Dublin, possessed, in addition to the usually described epiphyses, a small epiphysis for the gluteal ridge or third trochanter. The subject was a male of about seventeen years of age, and unfortunately the left femur only was preserved. The epiphysis for the third trochanter is a flat narrow scale of bone placed with its long axis corresponding with that of the shaft of the femur. Its upper end corresponds to the level of the lowest part of the small trochanter, while its lower end lies just above the groove so often present in the region of insertion of the gluteus maximus muscle (see 2 of figure). The epiphysis measures 25 mm. in length, and is 12 mm. across in its widest part. I have made an examination of a large number of femora in the Anatomical Department of Trinity College, and also of a number of young and adult bones kindly lent for the purpose by Prof. Cunningham from his private collection, and have found similar epiphyses in two other young bones. One of these (3 of figure) is the left femur of a boy of fifteen years; the other (1 of figure) is the right femur of an older individual, but the sex is not noted. In neither of these latter specimens is the epiphysis of the third trochanter as extensive as in the femur first described. In one, indeed, it is not more than 5 mm. in its longest diameter, while in the other it measures about 10 mm. In both cases its position corresponds with that described in the first case, its upper end being placed on a level with the lower part of the small trochanter. Although I have only been able to prove the presence of this epiphysis in the three femora figured, yet the appearance of the upper part of the gluteal ridge in a number of other bones leads me to believe that, in many cases at all events, this part of the bone is added to the femur as a special epiphysis. OSSIFICATION OF THE THIRD TROCHANTER IN MAN. 503 With regard to the ossification of the third trochanter in those lower animals which possess one, I have been unable in the literature at my disposal to find definite information. An examination of a number of skeletons, however, shows that a special centre may be present for it. This is easily demon- strated in the rabbit, and another rodent.-Cmalogenys paca- preserved in the Natural History Museum, Trinity College, affords a beautiful example of the same fact. In the rabbit, the special centre simply forms a thin scale over the strongly marked third trochanter, while in Caslogenys it appears to form the whole third trochanter, which is here not so strongly (1) (2) (3) The upper parts of three young femora, to show the epiphysis for the 3rd trochanter (3rd T.). developed. Among Perissodactyla, the tapir possesses such a centre,l and in the horse, although the usual descriptions do not take note of it, still Franck, in his Anatomie der Havumes ethiere, describes a centre for the third trochanter of the horse. It is, 1 Specimens showing this are preserved in the Natural History Museum, Trinity College, Dublin, and also in the Museum of the Royal College of Science, Dublin. 504 OSSIFICATION OF THE THIRD TROCHANTER IN MAN. however, he says, sometimes absent. I am indebted to Prof. J. M'Fadgean for this information regarding the femur of the horse. On the other hand, Owen, speaking of the third trochanter of the rhinoceros, says "ossification sometimes ex- tends from the great trochanter to the third trochanter." I have had no opportunity of examining a young specimen of this animal, yet I would venture to suggest that in the rhinoceros also a special centre may be present both for the third and the great trochanter, and that they unite in some cases to form a single epiphysis. This might give rise to the appearance of an extension of ossification from the great to the third trochanter. In one specimen of Cmlogenys paca that I have had an oppor- tunity of examining, the epiphyses of the great and third tro- chanters are in actual contact, although distinct. The fact that the third trochanter of man thus closely resembles in its mode of development that of lower mammals adds to it an interest, especially as the presence of this trochanter is stated to be a characteristic of the femora of higher rather than of lower races, and of man rather than of apes..