J. Playfair McMurrich 491 
afforded for the reference of the former muscle to the same layer as the 
latter one. 
Furthermore, it is worthy of remark that the occasional fusion of 
the abductor with flexor brevis quinti digiti in man, noted by Mac- 
alister and Le Double (1897) speaks in favor of a close phylogenetic rela- 
tionship between the two muscles. 
4. The opponens quinti digiti—This muscle also stands in intimate 
_ relationship with the flexor brevis, with which, as Le Double states, it is 
in general more or less related, and it may also unite with the abductor. 
I regard it as part of the flexor brevis superficialis and probably a de- 
rivative of the abductor quinti digiti of the lower mammals. Brooks 
(1886) refers it, in part at least, to the adductor layer, but this may 
probably be interpreted to mean merely that it is not to be considered 
part of the epineural musculature, the reason for such an interpretation 
being that which has already been adduced in speaking of the flexor 
brevis quinti digiti. 
5. The flexor brevis pollicis —This muscle has been the subject of a 
good deal of discussion, which has resulted in the establishment of the 
fact that, as usually understood, the muscle is really a compound struc- 
ture, including elements from different layers; the exact significance of 
the different elements is yet open to discussion, however. 
The description of the muscle given by Albinus has served as the basis 
for the accounts given in many and especially the English text-books, 
even the most recent of these, with the exception of that edited by Cun- 
ningham (1902), adhering to the original limitations of the muscle. 
According to this there is recognized in the muscle a cauda prior vel 
exterior, the outer head of the English texts, and a cauda posterior vel 
inferior, the inner head, which is again composed of three divisions. A 
very different view was that of Cruveilhier, who limited the term flexor 
brevis pollicis to that portion of the muscle which inserts into the outer 
side of the thumb and referred the rest to the adductor. Henle (1871), 
again, following Sdmmering, regarded the outer head as part of the 
abductor and the greater part of the inner head as belonging to the 
adductor, confining the term flexor brevis to a small slip which divides 
to be inserted into both the ulnar and the radial side of the proximal 
phalanx and corresponds to the second and third divisions of the inner 
head of the English texts. Furthermore, Henle called attention to the 
existence of a deeper head arising mainly from the first metacarpal and 
inserting into the ulnar side of the proximal phalanx of the thumb, 
regarding it as the true first palmar interosseus, thus recognizing four 
of these muscles instead of the usual three. 
