204 The Phylogeny of the Forearm Flexors 
portion later on divides-into slips, which may be traced distally to their 
insertion into either side of a strong fibro-cartilaginous nodule occurring 
at the metacarpo-phalangeal joint. 
In the meantime the deep layer of the palmar aponeurosis has divided 
longitudinally into slips or tendons, which are the continuations of the 
thickenings already mentioned as occurring in it, and one of these ten- 
dons lies immediately below the median portion of each flexor super- 
ficialis slip passing to the digits under consideration (Fig. 10, pi). 
When the final division of the muscle slips occurs, the tendons derived 
from the deep layer of the aponeurosis pass ventrally between the two 
terminal slips of the muscle and unite with the superficial tendons, pass- 
ing on with them to be inserted into the base of the terminal phalanges. 
The flexores medii, compared with the superficiales, are small muscles. 
As they are traced distally that for the fourth digit (Fig. 9, F. B. M.'Y) 
hes over the palmar surface of the fourth metacarpal, being separated 
from it by the corresponding flexor profundus (F. B. P.'Y), while that 
for the third digit (F. B. M.”’), much smaller than the other, les rather 
to the ulnar side of its metacarpal. The muscle of the fourth digit 
divides longitudinally into three slips (Fig. 10, F. M., F. M." and 7), that 
upon the ulnar side uniting with the subjacent flexor profundus IV, 
while the median and radial slips insert into the metacarpo-phalangeal 
fibro-cartilage, the median one entering into close relationship with the 
underlying median portion of the flexor profundus (F. P.’). The muscle 
of the third digit, owing to its more ulnar position with reference to the 
axis of the digit, lacks a radial slip, dividing into only two portions 
(F. M. and 7), the more radial of which corresponds to the median part 
of the fourth muscle and like it unites somewhat closely with the under- 
lying portion of the flexor profundus, while the ulnar portion inserts 
independently into the ulnar side of the metacarpo-phalangeal fibro- 
cartilage. 
The points to which attention needs to be especially directed for our 
present purpose are (1) the splitting of the palmar aponeurosis into two 
layers by the origin of the flexores breves superficiales, (2) the formation 
of tendons by the deep layer of the aponeurosis, which, after the division 
of the flexores superficiales into their terminal slips, pass up between 
them to join the tendons from the superficial layer of the aponeurosis, 
and (3) the origin of the flexores breves medii from the under surface 
of the deep layer of the aponeurosis. 
‘Turning now to the reptilia one is at once struck by the fact that there 
is no strong aponeurotic layer covering the surface of the flexor brevis 
superficialis (flexor sublimis seu perforatus Auct.) (Fig. 11, F. B. 8.). 
