MOTION. 



457 



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 vertebrae. According to 

 Weber, its greatest twisting or torsion hori- 

 zontally is denved from the dorsal region. The 

 angles of flexion of the head and trunk in two 

 cases were as follows : 



Crown of die head Sternum and Sum of the 



and sternum. sacrum. two. 



1 147 83 230 



2 175 . . 85 . . 260 



Mean.. 161. 



245 



Of these two cases nearly two-thirds of 245 are 

 accomplished by the head and neck, and one- 

 tliird only by the dorsal and lumbar vertebrse. 



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 

 fulcrum 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 vertebra ; 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 ihejirst 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 M.ijmdic Phys., by Dr. Milligan, p. 175. 



ship, it will tend only (as Dr. Arnott has alrea- 

 dy pointed out) to convey an erroneous impres- 

 sion, both of its stiucture 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 i\\ejirst. order, 

 having ils 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 other in 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 vertebrae 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 



