12 H. ST. J. BROOKS ON THE 
brevis* arises from the fibula immediately below and under cover of the preceding 
muscle. Its tendon divides in the foot into two slips. One slip is inserted into the 
base of the proximal phalanx of the little toe. The other and longer slip joins the 
division of the tendon of the common extensor which is inserted into the little toe. 
The close correspondence of this insertion to the insertion of the ext. carpi ulnaris 
in the hand of the same animal is very noteworthy. 
Intermediate sector.—The ext. communis digitorum is placed rather more 
deeply than the muscles belonging to the tibial and fibular sectors. It arises from 
the lower part of the shovel-shaped process and from the upper part of the shaft of 
the fibula, and is divided longitudinally into two parts. The tendons from these 
two parts, however, keep close together and divide to be inserted into the four outer 
toes. The tendon of the inner part passes exclusively to the index digit. The 
tendon of the outer part is distributed chiefly to the three outer digits, but sends a 
slip to the index also. 
Deep stratum.—The eat. brevis digitorum is represented by a large and 
elongated muscle, which arises from the shovel-shaped process of the fibula under 
cover of the peronei. It ends in a strong tendon, which becomes flattened and 
expanded on the tarsus. This expanded tendon divides into five slips which pass to 
the five digits. The first slip joins the tendon of the outer division of the tibialis 
anticus, and is inserted into the ungual phalanx of the hallux. The second, third, 
and fourth slips join the deep surface and outer side of the tendons of the common 
extensor which are inserted into the second, third, and fourth toes. The fifth slip is 
inserted into base of the proximal phalanx of the fifth toe, under cover of the tendon 
of the superficial extensor. 
The external popliteal nerve passes from behind forwards, winds round the 
shovel-shaped process of the fibula, and divides into a superficial and a deep branch. 
The superficial branch passes downwards towards the integument to the dorsum of 
the foot, where it divides into a number of large branches, which unite in a plexiform 
manner with the ramifications of the internal saphenus branch of the anterior 
crural.t The deep branch passes under cover of the peroneal muscles, and supplies 
* « Extensor brevis digiti quinti,” Rueu. (Op. cit., p. 594.) 
+ These two parts appear like two distinct muscles in Mrcxet’s plate, op. cit. 
+ This arrangement is exceedingly well seen in Rueu’s figure (op cit. Taf. XXXII. Fig. 3). In this 
respect the foot of Ornithorhynchus departs more widely from Reptiles than the majority of Mammals, for 
the deeper or muscular division of the peroneal nerve (anterior tibial) does not supply the integument of 
the foot at all, whereas in other Mammals it takes a more or less considerable share in the innervation of 
the toes. In Ateles (RucE, Fig. 38) it gives branches to all the toes. The same abrupt departure from 
the reptilian condition is seen in the forelimb of Omithorhynchus. The posterior interosseus nerve 
(which supplies the integument of the dorsum of the hand and digits in Hatteria and Alligator) ends 
above the wrist, and the radial nerve constitutes the sole supply of the back of the hand. In a Gibbon 
(Hylobates agilis) I found the posterior interosseus nerve supplying the integument on the dorsum of the 
ulnar half of the index finger, the middle finger, and the radial half of the ring finger. Sir Wit1am 
Turner has recorded a somewhat similar condition occurring as a variety in the human subject, viz., a 
branch of the posterior interosseus nerve dividing to supply the contiguous sides of the index and middle 
fingers. (Journ. of Anat. and Phys., Vol. VI., p. 105.) 
