380 
On the Sesamoids of the Knee- Joint 
The tendon of the popliteus contains a very strong fibro-eartilaginous fabella, which is placed 
above the head of the fibula to which it is attached, and on which it moves freely. It is with this 
fa.bella that the large gastrocneniial fabella articulates. 
In another paper* Parsons notes that in the koala,, the phalanger, the opossum 
and the Tasnianian Devil the fleshy part of popliteus has acquired an attachment to 
the fibula while the tendon is converted into an extra external lateral ligament. 
According to the views expressed in this paper it may be anticipated that the 
primitive marsupial would have a fused parafibula, and accordingly the later tendon 
of popliteus would be a femoro-fibular ligament and popliteus would arise from the 
fibula. From this standpoint it is not surprising that on the break-up of the fibular 
crest certain portions of popliteus should still be attached to the fibula rather 
vestigially than as an acquirement. 
We dissected a specimen of Koala {Phascolarctus cinereus) and found the para- 
fibular sesamoid articulating with the fibula : see our Plate XXXII, Figs. 82 a and 
82 h. The fleshy portion of popliteus was attached to the fibula and the popliteal 
tendon ran from the antero-proximal corner of fibula to popliteal groove of ex- 
ternal condyle. Also tendinous fibres from the fleshy portion on fibula cross the 
posterior surface of the fibula and mingle with the tendinous fibres of gastrocne- 
mius, which run from the sesamoid to the external condyle. 
No plantaris was discoverable. Young in his paper on the "Muscular Anatomy 
of the Koalaf " does not refer to popliteus or plantaris, but states that gastrocnemius 
is well developed and arises by two heads in the external of which a fabella exists. 
We again urge that this parafibular sesamoid should not be described as & fabella. 
We think that for the whole genus Macropus we ought to consider that we are 
dealing with a parafibular structure and not a lateral gastrocneniial fabella. We 
have not been able to examine a case of Petrogale xanthopus, but Parsons' descrip- 
tion seems in accordance with the above interpretation, if we replace his term 
fabella by parafibular sesamoid. Thus he saysj : 
The knee-joint like the hip is never fully extended, the capsule is strengthened by strong- 
lateral ligaments ; the external lateral runs downwards and backward from the external condyle 
to the front of the head of the fibula ; its anterior fibres are continuous with the origin extensor 
longus digitorum. The popliteus also performs the function of an external lateral ligament, since 
it is attached to the femur and fabella, and below to the external semilunar cartilage and 
the head of the fibula. During extreme flexion which is the usual position of the joint, t\\e fabella 
articulates with the head of the fibula. 
Judging from the knee-joints of other specimens of Macropus with which we 
are more familiar, we should believe that the parafibular sesamoid was always in 
contact with the fibula and that in cases of extreme flexure its head may come in 
contact with the postero-lateral border of the femur. We do not think that like an 
* " Joints of Mammals compared with those of Mim,'' Journal of Anatomy and Physiology, Vol. xxxiv. 
p. 313, 1900. 
, t Journal of Anatomy and Physiology, 1882, p. 237. 
+ "On the Anatomy of Petrogale xanthopus compared with that of other K&ngaxoos,'" Proceedings 
Zoological Society, London, 1899, p. 690. 
