MUSCULAR SYSTEM 603 
———. 
spini-humeralis and M. coraco-humero-brachialis would also fix and, 
moreover, in a measure describe the muscle ; but the application of 
these old names is not always easy, as shewn by the M. supra- 
coracoideus of Birds which really is the modified M. supra- 
spinatus of Man, and has been called the M. pectoralis minor, M. 
p. secundus, M. p. medius and M. subclavicus ; while the M. caud- 
ilio-femoralis figures as M. adductor femoris, M. gemellus, M. 
pyriformis and M. femoro-caudalis—the last being wrong and _ in- 
appropriate in more than one way—that is to say, it has been 
mistaken for at least three distinct muscles, and since the nerve- 
supply has not been ascertained by the writers who employ those 
names, it is generally doubtful which piece of flesh is intended to 
be described. Our knowledge of the homologies of avine Muscles 
may now be regarded as fairly settled by the present writer in 
Bronn’s Thier-reich (Vogel, pp. 91-325), thanks to the previous 
labours of Alix, De Man, Fiirbringer, Retzius, Rolleston and 
Riidinger, and it is based upon their nerve-supply and a study of 
their origin and insertion in a great number of different Birds. 
The taxonomic value of Muscles is theoretically great, but 
very limited when put toa practical test. Most of them cannot 
be understood unless the whole group to which they belong be 
examined, and the study of their correlations is a very complicated 
problem. ‘To pick out a few of the most variable muscles of the 
leg, and to arrange Birds according to their mere presence or 
absence, without regarding intermediate stages, is an easy but 
scarcely serious mode of investigation, and there is no wonder that 
systems built on such simple notions broke down. ‘There is no 
reason why a dozen different kinds of Birds should not have lost 
the same muscle at different times and independently of each 
other, and that other kinds may not lose it in future if its function 
be no longer required or can be fulfilled by some other combina- 
tion. Similar conditions may possibly have abolished 3 out of 
‘the 4 famous thigh-muscles in Cypselus, Trochilus, Striges, Fregata 
and Accipitres, and identical circumstances have caused Dicholophus 
and Serpentarius to assume the same ‘“ myological formula,” which 
in this case means only the loss of the caudal portion of the M. 
eaud-ilio-femoralis! It is certain that similar muscular combina- 
tions in two or more Birds do not necessarily mean relationship, 
while on the contrary similar requirements are often met in 
similar ways, that is to say the respective organs are ‘‘isomor- 
phous” if in two Birds they are modifications of one and the 
same substratum of the same previous condition, but if identical 
requirements were in both Birds reached after they had already 
differed in their substratum, the later requirement would be 
differently met, and the results would be no longer isomorphous. 
Thus if in a descendant of the Passeres the hallux became reduced 
