MECHANICS OF LOCOMOTION 



729 



oL.ucci. 



angle of insertion the greater will be the distance tion points enable us in some measure to conceive 

 traversed by the insertion point in one second of the cooperative action of many muscles of varying 

 time. From this it follows that if muscles contract lengths, speeds, and insertion angles, which may all be 

 at different rates the angles of insertion may be so inserted at different points on a single bone and yet 



complete one cycle of movement in nearly the 

 same time. It is also clear that in some muscles, 

 as in the flexors of the digits, great speed may be 

 developed at the beginning of a movement and 

 great power with slow speed at the end, and vice 

 versa. 



Certain of the limb muscles (such as the coraco- 

 brachialis) are wrapped around spirally warped sur- 

 faces, and many of the joints between the segments 

 (notably the astragalotibial joint) form segments of 

 spirals, so that the mechanical advantages of the 

 spiral wedge, or screw, are secured. By the proper 

 regulation of the pitch of the screw either great 

 "power" or great "speed" can be attained. For ex- 

 ample, in extreme graviportal types the upper artic- 

 ular surface of the astragalus is very gently inclined 

 downward and forward, corresponding to a wedge or 

 screw of low pitch. In this way the very heavy load, 

 represented by the pressure of the tibia upon the 

 astragalus, is easily pushed up the gently sloping 

 articular surface. In cursorial types, on the other 

 hand, the upper articular surface of the astragalus 

 faces more directly forward, and the pitch of the 

 screw is relatively high. Under these conditions a 

 relatively great expenditure of muscular energy is 

 required, but the movement of the tibia upon the 

 0, 7) and in a typically graviportal form (Ib, the astragalus is correspondingly rapid. 

 with narrow angles of insertion («', 0', y'). The entire complex of the skeleton, of the muscula- 



FiGURE 664. — Angles of insertion of certain extensor muscles 

 to the pelvis and femur in the standing pose of a typicallj' 

 cursorial form (A, the horse) with relatively wide angles of 

 insertion {< 

 mastodon) 



After Gregory, 1912.1 ture, and of the organs for developing and directing 



The heavy black lines represent the general direction of the muscles. The broken loCOmotivC energy is adapted in CUrSOrial auimals for 



leaping and fast running, in graviportal animals for 



the evenly maintained forward sweep of the walk and 



amble. The elongate feet and bent limbs of cursorial 



forms when suddenly straightened out act more or 



lines represent the radii of movement of the insertion points. In B the angle ( 

 is probably too small — W. K. Q. 



adjusted as to keep constant the distance traversed in 

 one second by the insertion point. The angle of inser- 

 tion and c o n s e- 



quently the recip- 

 rocal relations of 

 speed and power 

 change as the mus- 

 cle contracts. In 

 the case of the gas- 

 trocnemius the angle 

 of insertion de- 

 creases as the heel 

 is raised from the 

 ground; in the case Figure 665 



Planes of astraga/o-t/'b/a/ facets 



P/anes of 



astraga/a - 



na\//cc//ar faceis 



-Graviportal adaptation in the astragakis. Backward shifting of the plane of the astrag- 



alotibial facet from an angle of 82° in Ursui to an angle of 26° in Elephas, the plane of the 

 astragalonavicular facet being taken as horizontal. After Osborn, 1900.148 



Ur, Ursus: Ph, Phenacoiiis: Co, Coryphoion: Br, Brontops; Ui, Uintalherium: El, Blephis. 



of the flexors of the 

 digits the angle of 

 insertion and conse- 

 quently the "power" increases and the speed decreases 

 as the muscles contract. 



These variable relations of the rate of contraction, 

 of the angle of insertion, and of the speed of the inser- 



less after the manner of a catapult in hurling the 

 relatively light body forward. The very short feet 

 and straight limbs of graviportal forms act as long- 

 armed levers and low-pitched screws, which slowly 



