No.2.) THE HARD PARTS OF THE MAMMALIA. 211 
where the greatest weight of the body is supported, we are 
at once directed to strains as their cause. We cannot trace 
them to vertical strains, since the semi-cylinders and their em- 
bracing channels are perfectly horizontal, and not bent so as 
to permit vertical flexure of the column. They are not due to 
lateral curvature of the column, for then each cylinder would be 
convex externally in the longitudinal direction, or perhaps much 
abbreviated anteroposteriorly. They appear to have been caused 
rather by longitudinal torsion of the column. 
Figure 45.— Diagrams representing the movements of the vertebral column in 4, 
the pace; 4, the run; and C, the trot or walk. 
The mechanical cause of this torsion is as follows. Viewed 
from the standpoint of their mechanical effects on the skeleton,! 
the gaits of animals may be referred to three types. The first 
is that in which the both feet of a pair strike the earth together, 
and in alternation with those of the other pair when it is used. 
1 Dr. Harrison Allen (AZemotrs for a Memoir on Animal Locomotion, 1888, p. 37) 
divides the gaits of quadrupeds differently, and apparently with reference to the order 
of innervation of the limbs in motion; that is, in accordance with their mode of co- 
operation. From this point of view Dr. Allen refers all gaits to two types, syachiry 
and heterochiry. In the former “ the right and left foot of a single pair act together,” 
whether synchronously or alternately. In hetrochiry, locomotion is accomplished 
by “ combination of hind and fore feet,” 
