MUSCULAR SYSTEM 121 



In general, the principle holds good that those muscles 

 are most subject to variation which can be dispensed with 

 without disturbance or disadvantage to the organism as a whole, 

 either because they can be easily replaced by other muscles, 

 or because they have only a subordinate part to play. In 

 illustration of this I would merely refer to the pyramidalis, 

 the abortive caudal muscles, the muscles of the pinna, the 

 palmaris and the plantaris, the vestigial character of which 

 clearly points to their ultimate complete suppression. 



Eesearch has shown, however, that it is not only to the 

 retrogressive tendency of the muscles that variation is due, but 

 that variation may in some cases indicate a tendency to 

 progressive development. The best example of this is afforded 

 by certain flexor muscles, and by the flexor longus pollicis, and 

 the gluteus magnus. 



A third kind of variation occurs, in those cases in which a 

 tendon may return to former points of insertion on neighbouring 

 bones, e.g. the rectus abdominis is occasionally inserted on to the 

 more anterior ribs. And to the same category belong the \ 

 splitting off of the abductor hallucis from the tibialis anticus, 

 which occurs in very varying degrees. 



All these cases, which must be denominated reversionary, 

 indicate the extraordinary tenacity with which certain 

 peculiarities persist and are repeatedly passed on from one 

 generation to another. This power of reproduction must, however, 

 necessarily grow weaker, as an organ in course of time loses its 

 original functions in adaptation to new ones. As a consequence 

 of this, attempts at reconstruction necessarily become more and 

 more imperfect. 



The same is the case with many other muscles (e.g. the 

 sternalis, levator claviculse, latissimo-condyloideus, and epitrochleo- 

 anconseus) which now only rarely occur in Man, and which, when 

 they are present, furnish important indications of a long-past 

 period in the development of the human race. 



There is no good ground for doubting the possibility of the 

 hereditary transmission of muscular anomalies, although, as Testut 

 rightly remarks, the difficulty of obtaining material for a direct 

 proof is evident. The difficulty in this case is greater than in 

 that of mere external variation, such as pigmentation, different 

 coloration of the opposite eyes, abnormal hairiness, birth-marking, 

 polydactyly, and others akin to these. 



It is reserved for future investigators to add to our as yet 



