76 THE EVOLUTION OF MAN 



It may be that at some time in man's ancestral history this rib 

 regularly existed but now it occurs only occasionally as a re- 

 version to an ancestral type. Its disappearance may perhaps 

 be associated with the assumption of the upright gait, with the 

 resulting dragging down of the viscera by gravity. A tendency 

 to the loss of more ribs is seen occasionally in the partial 

 deficiency of the first rib and in the absence of the twelfth. 



Rarely a fourth molar tooth occurs in man, who regularly 

 has only three. Some of the lower mammals have four such 

 teeth. It is possible that the fourth molar in man, when it ap- 

 pears, is a reversion to the former condition. This tooth is 

 more likely to occur in the primitive races and presumably 

 occurred still more frequently in those prehistoric races which 

 are known only by their fossil bones. 



In about four per cent of persons a small muscle exists in the 

 front of the thorax, parallel with the sternum, known as the 

 sternalis. Anatomists have been much puzzled for an explana- 

 tion of the occasional presence of this muscle. One theory is 

 that it is part of the pectoral muscle which in some unknown 

 way has become twisted through ninety degrees. That this 

 may sometimes be the proper explanation is indicated by its 

 receiving a branch of the same nerve which supplies the pec- 

 toral muscle. A second theory is that the sternalis is a recur- 

 rence of part of the subtegumentary muscle which moves the 

 skin, and which is still present in many mammals but has been 

 largely lost in man. A third explanation is as follows: in the 

 amphibians a longitudinal muscle runs from the pelvis along 

 the front of the entire body. Although the abdominal part of 

 this is present in man as the rectus muscle, the thoracic part is 

 absent, having been crowded out of existence, we believe, by 

 the enlargement of the pectoral muscles following the in- 

 creased use of the upper extremities for prehension. Some 

 authorities hold that the sternalis, when present, is the thoracic 

 portion of the rectus. An occasional nerve supply from the 



