THE ASCENT OF MAN 455 



great uiusde of man's calf, the triceps extensor surw, is formed by the 

 welding together of some four muscles separate in many lower forms. 

 Varieties are found in man showing all grades of separation in these 

 elements. One of the muscles, the plantaris, was formerly a great 

 flexor of the toes, the plantar fascia representing its former distal ex- 

 tent. Like the palmaris of the arm it lost its original function by the 

 welding of the fascia to the bones to secure the plantar arch, and its 

 functions being then assumed by other muscles it began to dwindle, 

 and is now represented by a mere vestigial rudiment of no functional 

 value. It is well known tbat the lower races of men have smaller 

 calves than Europeans. Again, it should be noted that as the erect 

 position is assumed tlie muscles required for the flexion and independ- 

 ent action of the toes become reduced in character. A comparison 

 with other forms shows that some of the small muscles now confined to 

 the region of the foot formerly took their origin higher up, from the 

 bones of the leg. Losing in functional importance, they have dwindled 

 in size and gradually moved downward. 



The great gluUei muscles of the buttock find their highest develop- 

 ment in man. They are subject to similar variations. Certain muscles 

 of this region, normal in apes, are occasionally found in man: a sepa- 

 rate head of the great glutseus, derived from the ischium, and the 

 scansorins or climbing muscle that assists the great flexor of the 

 thigh (the iUo-psoas), may be mentioned. 



The enormous size and complexity of the muscles of the back in man 

 are well known. The erector of the spine fills up the vertebral grooves 

 and sends up numerous tendons along the back like stays supporting 

 the masts of a sliip. The mass of this muscle is comparatively less 

 in anthropoid apes. 



Notwithstanding all these powerful muscles, it would be impossible 

 to retain the erect position for any great length of time were we to 

 depend upon them alone, for it requires (as before stated) a great ex- 

 penditure of force to keep a muscle in active use. It becomes rapidly 

 fatigued and then loses its power, as any one may prove by standing 

 in any constrained position, even " in the position of a soldier," for half 

 an hour. To provide against this, a beautiful arrangement of joints 

 and ligaments has been developed. 



When in the erect attitude the ankle-joint is so arranged that its 

 bones are in a position of greatest stability and the center of gravity' 

 is so adjusted that it falls directly upon it. This reduces to a minimum 

 the amount of muscular force required to keep the body erect. At the 

 knee the center of gravity falls a little in front of the axis of the limb, 

 and the back and sides of the joint are i)rovided with check ligaments 

 or straps that hold the joints locked in a position of hyper-extension, 

 so that no muscular force whatever is used to maintain it. These liga- 

 ments are regressive structures, being vestiges of former insertions of 

 muscles near the joint. At the hip a similar condition occurs, the 



