THE HUMAN SPECIES 



Regressive muscles in the human trunk are the musculi 

 scaleni (see Fig. 40), and the m. triangularis sterni, the m. rectus 

 abdominis (see Fig. 41) and pyramidalis, the vertebral and 

 dorsally situated remnants of those muscles which in the caudate 

 mammals serve to move the tail. Occasionally in man the m. 

 caudo-femoralis is still found, a muscle which plays a great 



part in many of the 

 caudate mammals. All 

 the remaining caudal 

 muscles of the mam- 

 mals are to a greater 

 or lesser degree de- 

 generate in man, and 

 the m. levator ani (or 

 diaphragma pelvis) in 

 man, and the anthro- 

 poids, must be con- 

 sidered as a remnant of 

 the m. pubo-coccygeus 

 and ileo-coccygeus. 



Darwin has drawn 

 attention to numerous 

 muscles of the human 

 arm and hand which 

 show a strong tendency 

 to vary so as to re- 

 semble the correspond- 

 ing musclesin the lower 

 animals. He attributes 

 these variations to re- 

 version (atavism), and 

 in support of his opinion refers to the views of Wood, who con- 

 siders that notable departures from the ordinary type of the 

 muscular structures run in grooves or directions, which must be 

 taken to indicate some unknown factor, which is of much im- 

 portance to a comprehensive knowledge of general and scientific 

 anatomy. 



Indeed, many a remarkable variation in the human muscles 

 is only to be understood when regarded as a remnant of a 



FIG. 41. M. rectus abdominis. 

 (Heitzmann-Zuckerkandl.) 



