MUSCULAR SYSTEM 603 



spini-liumeralis 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 Thieweich (Fogel, 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 to a 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. 

 caud-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 



