160 
On the Sesamoidii of the Knee- Joint 
While Gruber asserts the universal absence of the mesial fabella in man, he 
draws attention to the fact that in other mammals the nature of the mesial is 
differentiated from that of the lateral. Lateral fahellae may always occur without 
mesial; or the mesial may occur occasionally; but there is no type of life in 
which the mesial always occurs without a lateral fahella. This differentiation 
both in man and other mammals between mesial and lateral fabellae is very 
significant and should find its expression in any theory of origin*. Among other 
points dealt with by Gruber are the shapes and sizes of the fabellae. He divides 
them into 20 shapes to which he gives mathematical terminology, the most 
common being tetrahedra, cones, half ovals, four-sided pyramids, oval or circular 
plates or discs. There is no mathematical accuracy in these descriptions, see for 
example his Table IV, Figs. 9 and 10 (which he calls a three sided pyramid 
form !), pp. 30-31. All that we can say is that the fabellae are very variable in 
shape, and that they may be pyramidal as in the marsupial type, or lens-form 
even to spherical as in the apes, and this only in the roughest resemblance. The 
"base" is usually smooth. It seems to us that the shape largely depends on the 
number of muscle-attachments of which the fabella is in any case the node, but 
this point is not emphasised by Gruber. On pp. 35-38 certain measurements are 
given. The greatest weight of any human fabella was 0"847 gr. and the smallest 
weight 0'009 gr. In the greatest tetrahedral form the height was 13"5 to 14 mm. 
and the base is given as 10 mm. diameter. 
We now come to a most interesting consideration of the locus of the external 
fabella in man. Gruber gives three situations for the gastrocnemic ossicles in 
man. The first of these (p. 38) is the sulcus popliteus externus ! Now if the 
reader will examine the case described by Gruber on his p. 38 and illustrated in 
Fig. 1 of his Table If (see our Plate V, Fig. 10) I think he will agree that it 
is no fabella at all, but a true cyamella and probably most of the pyramidal 
fabellae described by Gruber are really cyamellae. Now both fabella and cyamella 
occur simultaneously in the knee-joints of many genera and are frequent in the 
apes. Hence it is remarkable that Gruber should have overlooked this important 
distinction, and confused a sesamoid occurring in the depth of the sulcus popliteus 
externus (p. 71) with a fabella of the tendon of the M. gastrocvemius. Gruber 
even tells us that the hump due to this sesamoid can be so large that it can be 
mistaken for an exostosis ; in the extended leg the hump can be so great as to be felt 
or even seen\ (p. 71). Now there is, perhaps, more I'eason for this confusion than 
* Confirmation of this differentiation of origin of the fahellae is to be found (i) in the fact that the 
two fahellae are not equal in size. It is possible that in the apes the mesial may occasionally be larger, 
but in all other species it is smaller and frequently much smaller, (ii) the position of the fahellae is often 
different, (iii) ossification of the external ./V(6e?Zrt begins and ends sooner, (iv) the mesial more often than 
the lateral fails to ossify, and (v) the microscopic structure may be different. 
t This figure represents what is almost certainly a cyamella embedded in M. gastrocnemius, Ligamentum 
popliteum and short external lateral ligament. 
J " Der Hocker von dem Ossiculum oder der Cartilago wird 2-6 — 1"2 cms. (in medium 3-325) iiber der 
Spitze der mittleren, hochsten Zacke als dem hcichsten Punkte des capitulurn fibulae gefiihlt oder 
sichtbar " (p. 39). 
