40 THE VOYAGE OF H.M.S. CHALLENGES. 



internal condyle. Its lower margin, therefore, reaches lower down than that of the 

 adductor brevis, and it is, in like manner pierced by the femoral artery (PL IV. fig. l,f.a.). 

 The quadratus femoris is in apposition with its upper border, whilst fused with it 

 posteriorly are the lower fibres of the ischio-femoral muscle. 



In the Cuscus (PI. V. fig. 4) there are only two adductors present, viz., the adductor 

 brevis and the adductor magnus. The absence of the adductor longus is rendered evident 

 by the fact that the obturator nerve lies upon the superficial aspect of the adductor brevis. 



The adductor brevis (e) is much the larger of the two muscles, and it completely hides 

 the adductor magnus from the front. It has a curved origin from the base of the marsupial 

 bone, and from the margin of the pubic symphysis, under cover of the gracilis (/). Its 

 insertion is the same as the corresponding muscle in Thylacinus, and, Hke it, it is pierced 

 by the femoral artery (e 1 ). The adductor magnus (d) is a thick fleshy strap, which arises 

 from the arch of the pubis and the ischial tuberosity, and is inserted into the posterior 

 aspect of the shaft of the femur in its lower third. It is not perforated by the femoral 

 artery. The two muscles are easily separated from each other. 



The triceps adductor is subject to considerable variations in the Marsupialia. Thus 

 Macalister x records that the three portions are quite distinct in the Wallaby and Giant 

 Kangaroo, whilst the adductor brevis and the adductor magnus are partially, or it may 

 be completely, blended in the Opossum, Sarcophilus, Wombat, and in Phalangista. 



Posterior Aspect of Leg. 



The superficial muscles on the back of the leg are the gastrocnemius and the plantaris. 

 In all probability the absent soleus is blended with the outer head of the gastrocnemius. 



Gastrocnemius. — This muscle in Thylacinus arises by two very large and powerful 

 heads. The outer head which contains the soleus is much the larger of the two, and it 

 arises by two parts — (l) by a fleshy process fixed to a large sesamoid bone placed upon 

 the posterior aspect of the head of the fibula, and from a powerful ligament which binds 

 the upper part of this bone to the back of the external condyle of the femur ; (2) by a 

 tendinous slip attached to the outer aspect of the external condyle of the femur. The 

 external popliteal nerve passes forwards between these slips of origin. The inner head, 

 springs not only from the back of the internal condyle, but also from the whole breadth 

 of the popliteal surface of the femur at the same level. The heads of the gastrocnemius 

 join each other very shortly after their origin and the tendo achillis is inserted into the 

 lower part of the posterior surface of the os calcis. 



In the Cuscus the two heads of the gastrocnemius are separate throughout their entire 

 length. The muscle therefore is present in the form of two distinct factors. The inner 

 head (PI. V. fig. 2, b) arises from the back of the internal condyle of the femur ; it derives 



1 Ann. and Nat. Hist., vol. v., 4th series. 



