THE ASCENT OF MAN. 451 



modern times thau iu those of the stone age. As the torsion increased 

 some provision became necessary for carrying the hand easily across 

 the body to the month. This was effected by the inclination of the ar- 

 ticular surfaces of the elbow-joint already mentioned. 



Many movements of the arm in man are produced by muscles acting 

 upon the shoulder-blade or scapula. As the hand was turned outward 

 and a wider range given, these increased in extent and importance, and 

 the scapula accordingly widened out at its vertebral border iu order to 

 give a more extensive attachment for muscles. In order to accurately 

 estimate this change the ratio of the breadth to the length of the 

 scapula is taken. This ratio, called the scapular index, is highest 

 among the white races, less in the infant, iu negroes, and in Austra- 

 lians, and still less in anthropoid apes. It is significant also that the 

 vertebral border of the scapula is the last to form in the fcetus. We 

 have therefore three modifications — the torsion of the humerus, the 

 inclination of its lower articular surface, and the scapular index — all 

 depending upon each other, all varying together ^Mri passii, and all 

 showing a progressive development both in the individual and the race. 



Muscle is composed of one of the most highly organized and expen- 

 sive tissues of the body. Unless fed constantly with a great supply of 

 blood to keep up its active metabolic changes, it quickly wastes, func- 

 tional activity being absolutely necessary to its proper maintenance, as 

 any one knows who has seen how rapidly the muscles of an athlete 

 diminish when he goes out of training. If from accident or change of 

 habit its use altogether ceases, its protoplasm is gradually removed, its 

 blood supply diminishes, and it shrinks to a mere band or sheet of 

 fibrous tissue. Changes of function may therefore affect the form of 

 muscles, one j)ortion becoming tendinous or fascia-like : may even cause 

 them to shift their places, by inducing a development on one side and 

 an atrophy on another, or to disappear altogether, being replaced by 

 fascia or ligament. A similar regression may take place in bone and car- 

 tilage a high-grade, actively metabolic tissue, difficult to maintain, being 

 replaced by a low-grade one comparatively slow to change. It is there- 

 fore not unusual to find that muscles, bones, and cartilages i)erforining 

 important functions in some animals are represented by vestigial struc- 

 tures in those higher in the scale. Our conclusions on this subject are 

 confirmed by finding occasional instances where the hereditary ten- 

 dency has been greater than usual and the parent form is re-produced 

 more or less completely in the higher animal. The palmar fascia at the 

 distal end of the pahndriH longns, to which allusion has been made, 

 represents a former muscular portion, relics of which probably remain 

 as some of the small thumb muscles. 



Another interesting instance is the epitrochleo-anconeun, a small 

 muscle at the elbow joint, used in apes to effect a lateral movement of 

 the ulna upon the humerus. In man the ulna has become so shaped 

 that the lateral movement is almost wholly lost, and the muscle has 



