50 
17, ¢.7.m.). This second deep extensor arises from the posterior 
aspect of the shaft of the ulna by a narrow, pointed, fleshy belly, 
which extends upwards on the bone as far as the base of the 
olecranon, from which its highest fibres arise. The muscle covers 
the attachment to the shaft of the ulna of the anconeus externus, 
and in the distal part of the forearm it is directed obliquely to- 
wards the radius under cover of the extensor carpi ulnaris, so as 
to enter the same fibrous compartment of the dorsal annular 
ligament which transmits the tendons of the extensor communis 
digitorum. Still running obliquely its tendon passes beneath the 
tendons of the latter muscle, and comes to lie by their radial side. 
It then divides into two slips, the stronger of which passes to 
the dorsum of the index, where it is inserted, partly by lateral 
expansions into the base of the proximal phalanx and _ partly by 
a direct prolongation of itself into the base of the ungual phalanx. 
The other (weaker) division of the tendon is inserted into the 
radial of two slight tubercles on the prominent dorsal projection 
of the base of the ungual phalanx of the third digit. 
M. extensor secundi internodi pollicis (extensor pollicis longus) 
is unrepresented, or rather the above muscle is the sole repre- 
sentative of the muscular mass from which the extensor secundi 
is typically differentiated. 
In Thylacinus and Phascogale, according to Cunningham, there isa 
single muscular mass sending tendons to pollex, index and medius, 
which he designates as extensor secundi internodii pollicis. It is of 
course the equivalent of the muscle now under consideration, with the 
addition of a pollicial division. 
In Cuscus the author just mentioned found the same mass repre- 
sented by two distinct muscular factors connected respectively with 
pollex and medius. 
The origin of the compound muscle in T'hylacinus and Phascogale 
resembles that in Notoryctes. There “it springs from the radial side 
of the olecranon and from the upper third of the posterior border of 
the ulna.’’* 
Macalister} describes an extensor secundi internodii pollicis arranged 
“as usual” in Koala, but Young{ could find so such muscle in that 
animal, nor did he find any such indicator muscle as Macalister also 
describes “ giving a filmy slip to the pollex.”” Young expressly says 
that “the thumb has no special extensor of the phalanges beyond the 
slip derived from the common extensor.” 
Both authors describe an extensor secundus digitorum supplying the 
fourth and fifth digits, and the extensor medii digiti was conjoined 
with this in Young’s dissections, but separate in Macalister’s. The 
slips to the fourth and fifth digits Young holds to represent the 
extensor minimi digiti. It arises from the shaft of the ulna along 
with the extensor medii digiti (cf. page 44). 
I am strongly of opinion Young has described, as part of the ex- 
*iv., page 16. + xxviii, page 131. +t Ixxiis page 229. 
