oF ES eS ee ee 
MOTION. 
length, will be nearly three times greater. The 
greatest flexion of the trunk is in the plane of 
its mesial section, and results chiefly from the 
cervical and lumbar vertebre. According to 
Weber, its greatest twisting or torsion hori- 
zontally is derived from the dorsal region. The 
angles of flexion of the head and trunk in two 
cases were as follows: 
Crown of the head Sternum and Sum of the 
and sternum. sacrum. two. 
Beteriie tO! oecsee OS occas. 2300 
DUeMeadiet Oo 5.600 85° -..\c¢20 260° 
Mean.. 161° 84° ...... 2450 
Of these two cases nearly two-thirds of 245° are 
accomplished by the head and neck, and one- 
third only by the dorsal and lumbar vertebrz. 
The curved form of the spine, somewhat re- 
sembling an italic f, instead of being a cause 
of weakness, is, on the contrary, a source of 
strength and security; for forces acting verti- 
cally upon it are transmitted obliquely, thus 
diminishing their mechanical effect ; and the 
elasticity of the intervertebral tissue prevents 
the shock to which the bones would otherwise 
be subjected at every step in walking. With 
respect to the power of the vertebral column 
to sustain weight, if we regard it in conse- 
quence of its form and elastic intervertebral 
Substance as a spring with small flexures, it is 
capable of bearing a greater weight than if it 
were straight, in the proportion of the square 
of the number of curves plus one to unity ; 
that is, a weight sixteen times as great. Consi- 
dered as a whole, the vertebral column repre- 
sents a lever of the third order, of which the 
fulerum is in the axis of the articulation of 
the fifth lumbar vertebra with the sacrum: the 
power is the mass of muscles inserted into the 
sides of the vertebre; and the resistance, the 
weight of the head, soft parts of the neck, the 
thorax, and part of the abdomen.* 
In standing erect, the direction of the verte- 
bral column is perpendicular to the horizon. 
Weber found, by means of plumb-lines let fall 
on each side opposite the centre of the heads of 
the femurs on which the trunk is balanced in 
the erect position, that the transverse vertical 
plane intersected both ends of the spine, pass- 
- Ing through the atlanto-occipital articulation 
above and the sacro-lumbar below; he found 
also lying in the same plane the two mastoid 
processes and the centres of the ankle and knee 
joints. 
Viewed separately, each vertebra represents 
a lever of the first order, whose fulcrum is the 
next vertebra, on which it rests; the power and 
resistance are the muscles acting upon it in 
different points alternately. The spinal column 
of animals is a flexible lever, destined to move 
_ and support a multitude of organs, and to con- 
_ nect the more distant parts of the skeleton with 
each other. We have nothing in the structure 
of locomotive or other machines bearing the 
least resemblance to its mechanism; and if, 
with Sir C. Bell, we compare it to the mast of a 
* Vide Majendie Phys., by Dr. Milligan, p. 175. 
457 
ship, it will tend only (as Dr. Arnott has alrea- 
dy pointed out) to convey an erroneous impres- 
sion, both of its structure and its functions. 
In standing, the vertebral column transmits 
the weight of the organs appended to or sup- 
ported by it by means of the sacrum to the 
pelvis. The pelvis is a lever of the first order, 
having its fulcrum in the ilio-femoral articula- 
tions on the heads of the femurs; the power 
and resistance are the muscles acting anteriorly 
and posteriorly to the axis of rotation. 
In the erect position of the trunk the pelvis 
is inclined to the direction of the vertebral co- 
lumn. The angles, both of the superior and 
inferior margins, have been measured by Nae- 
gele and Weber. According to the former, the 
mean angle of inclination of the superior mar- 
gin to the horizon is 60°, and to the vertebral 
column 150°. The inclination of the inferior 
margin with the horizon 11°, and with the ver- 
tebral column 101°. The angle which the 
pelvis forms with the vertebral column permits 
the femurs to extend farther backwards, and to 
increase their range of oscillation. The magni- 
tude of the transverse diameter of the ring of 
the pelvis determines the distance of the heads 
of the femurs from each other, and throws the 
thighs sufficiently apart, to prevent their friction 
upon each otherin walking. The pelvis can ro- 
tate only on an axis perpendicular to the plane of 
the mesial section, when the body rests equally 
on both legs, but can turn horizontally upon 
the heads of either femur whilst the body rests 
on one leg. In standing, the pelvis is kept 
in equilibrio by several forces. The weight of 
the abdominal viscera, lying anterior to its axis 
of rotation upon the femur, tends to depress 
the pubes, and rotate the pelvis forward, while 
the weight of the vertebral column, acting pos- 
terior to that axis, tends to swing it back- 
wards. 
The weight of the vertebral column prepon- 
derates over the parts lying anterior to the axis 
on which the pelvis rotates, and consequently 
would require a considerable expenditure of 
muscular action to keep it in equilibrio on the 
femurs, but the obliquity of the vertical axis of 
the pelvis to that of the vertebral column 
throws the resultant of all the forces, acting 
downwards upon it, near the axis of rotation, 
which tends to preserve the balance; added 
to which, the muscles which draw the ver- 
tebral column backwards, having one of their 
points of insertion in the sacro-iliac section 
of the pelvis, tend to elevate it, and to neu- 
tralize their action on the vertebre in an 
Opposite direction. The principal agents in 
keeping the pelvis in equilibrio are the pow- 
erful muscles acting between it and the thigh. 
The pelvis receives the whole weight of the 
trunk and superposed organs, and transmits it 
to the heads of the femurs. It has two axes of 
motion, one of which is on the last vertebra, 
and the other in the ilio-femoral articulation ; 
in both cases it acts as a lever of the first order. 
The legs.—The leg moves by means of three 
joints, namely, the ilio-femoral, the knee, and 
the ankle. In the erect posture the first of 
these allows the leg to move only forwards, the 
