282 ANNALS NEW YORK ACADEMY OF SCIENCES 



the resistance to be overcome. 39 The end result, as it were, determines 

 the rate of contraction of the coordinated muscles. The effective regu- 

 lation and correlation of muscular action is obviously an extremely com- 

 plex function of the peripheral and central nervous systems. The force 

 and speed of contraction of a given set of muscles in a living animal at 

 a given moment are determined not only by many mechanical factors, of 

 which a few have been mentioned above, but also by the whole psychic 

 constitution of the animal and by the psychic effectiveness of the ex- 

 citing "motive." 



Fig. 4. — "GraviportaV adaptations for the walk and amble in the Mastodon 



Application of the Fobegoing Pkinciples to the Study of the 

 Limbs of Ungulates 



functional significance of the angulation of the limbs 



As noted above (p. 269), the straightness of the limbs in the Probos- 

 cidea and similar heavy-bodied animals was interpreted by Osborn in 

 1900 as "an adaptation designed to transmit the increasing weight 

 through a vertical shaft." While this is no doubt an incidental advan- 

 tage of the straightness of the limbs, it is probably not the chief teleo- 

 logical "object." From a consideration of the mechanical principles 

 governing the use of the limbs as compound levers (see'pp. 278-281) and 



J. Bukdon Sanderson, in Schafer's Text Book of Physiology, Vol. 2. p. 363. 1900. 



