REPORT ON THE MARSUPIALIA. 47 



Extensor brevis digitorum. — In the memoir by Ruge, to which reference has been 

 made, the author traces with great ability the history of this muscle. It belongs 

 originally to the peroneal group of muscles, and in the Monotremata it may be observed 

 to arise entirely from the fibula. Its progress as it passes down to the dorsum of 

 the foot can be traced step by step through the mammalian series. First one 

 belly descends, then another, and so on until the entire muscle gains the dorsum of the 

 foot, as in man. 



In Thylacinus the entire muscle arises from the fibula ; in the Cuscus the part belonging 

 to the medius and index has reached the dorsum of the foot, whilst that belonging to the 

 two outer digits still shows a fibular origin. 



In the Thylacine the extensor brevis digitorum is represented by three separate 

 fleshy bellies, all of which arise from the fibula under cover of the two peronei muscles. 

 The largest of these fleshy bellies springs from the upper third of the fibular shaft, and 

 its tendon joins the dorsal expansion of the extensor tendon of the little toe ; the other 

 two arise side by side from the middle third of the fibular shaft, and of these the posterior 

 belly gives rise to a tendon which divides to reach the extensor expansions of the index 

 and middle digits, whilst the anterior belly sends its tendon to the extensor expansion of 

 the ring toe. All these tendons accompany the tendons of the peronei muscles behind 

 the external maleolus. 



In the Cuscus the fibular portions of the extensor brevis digitorum are two in 

 number, viz., (l) a good sized muscle (PI. V. fig. 3, c.) which arises from the outer 

 aspect of the head of the fibula, under cover of the superficial head of the peroneus 

 longus ; the slender tendon of this fleshy slip passes behind the external maleolus, and 

 is inserted into the dorsal expansion of the extensor tendon of the minimus (c ( .) ; (2) 

 the second fleshy belly (PI. V. fig. 3, d) is subjacent to the preceding, and springs 

 from the middle third of the inner aspect of the shaft of the fibula ; its tendon passes 

 behind the external maleolus and gaining the dorsum of the foot is inserted into the 

 extensor expansion of the ring digit (d l . ) 



The pedal portion of the extensor brevis digitorum in the Cuscus (PI. V. fig. 3, 

 h) arises by a narrow tendon from the outer aspect of the os calcis. It is a very 

 minute fleshy band, which ends in two delicate tendons for the index and medius 

 respectively. 



In the Koala 1 the arrangement of the extensor brevis digitorum is similar to that of 

 the same muscle in the Cuscus. 



According to Ruge 2 the fibular portion of the muscle in Didelphys and Dasyurus 

 consists of two fleshy bellies, of which one sends a tendon to the fifth toe, whilst the 

 other supplies tendons to the second, third, and fourth toes; further, in Didelphys there 

 is a pedal portion which is devoted solely to the hallux and index. 



1 Young, he. cit., p. 41. " Loc. cit., p. 4G. 



