Karl Pearson and Adelaide G. Davin 
135 
anomalies. In other species some will only appear as hemisesamoids ; while in a 
third species, if they have been reported at all, it is pi'obable that they were really 
pseiidosesamoids. 
Now to obtain a large number of knee-joints for dissection in the case of a . 
particular species is practically impossible. In the case of dry material and skeletons 
the articulator may, perhaps generally does, preserve the fabellae, tar more rarely 
the cyamellae or lunulae*. Thus the actual presence of ci/amella or lunula conveys 
definite information, their absence practically conveys nothing. It is extremely 
hard to demonstrate an invariable absence, especially in the case of an anomaly 
which may have a very small percentage of occurrences like the lateral cyamella in 
man. When one anatomist asserts the existence of a sesamoid which nine others 
deny, there is no grave difficulty in reconciling their statements, if the anomaly be 
present in only a small percentage of cases, and none of these anatomists have had 
the chance of examining more than a few individual cases of the species in question. 
This is, we think, the quite adequate explanation of the contradictions we find in 
the early history of the subject, and forms the chief difficulty in applying our 
analysis of the sesamoids of the knee-joint to the problems of evolution. 
These problems are of the following character. Suppose we can classify those 
families in which certain sesamoids are (i) invariably present in the knee-joint, 
(ii) invariably absent, (iii) exist only as anomalies of a given order of rarity, will not 
such classifications throw light on evolutionary order ? For example if we confine 
our attention to the primates, there are families in which the fabellae are. both 
invariably present, others in which they are both invariably absent, others in which 
both are anomalous, and still another group in which the fahella lateralis is a true 
or orthosesamoid when it appears, and the fahella mesialis a hemisesamoid. Again 
the cyamella lateralis is invariable in certain monkeys, never appears in the gibbon, 
but is constant in the orang, and a rare anomaly in man. A similar series of state- 
ments can be made with regard to the presence or absence of lunulae in the 
primates. May it not be possible to thi'ow light on primate evolution from such 
considerations ? 
The problems with regard to sesamoids are not only rendered difficult of solution 
by reason of the amount of material requisite for adequate classification, but by 
the longstanding existence of opinions and dogmas with regard to them. 
The definition of the medical student that a sesamoid is any bone which it is 
unnecessary to study for examination purposes is, of course, an echo of the treatment 
of sesamoids in the current anatomical text-books, and that treatment has its 
historic evolution — broken only and for a short period by the discovery of the 
fabellae in man by Vesalius — fnmi Galen to the present day; the text-book 
anatomists from his day through the centuries to our own have echoed his phrase 
as to sesamoids— that of those bones " which are termed sesamoids from their 
* lu 15 dog skeletons in the Bionietric Laboratory, the fabellae are almost invariable, the cyamella 
has not been preserved in a single case. 
9—2 
