456 



MOTION. 



from acting with sufficient mechanical advan- 

 tage to extend the thigh as perfectly as in 

 man. Vicq D'Azyr * states, that in the Man- 

 drills the flexors of the leg are inserted lower 

 than the extensor muscles, and that they 

 oppose the perfect extension of the leg upon 

 the thigh, which renders it impossible for them 

 to stand upright for any length of time without 

 tottering. The Chimpanzee, the Ourang-Outang, 

 and some of the Gibbons, however, are ca- 

 pable of walking upon their posterior extre- 

 mities with the trunk nearly erect ; but the 

 pelvis being narrow, and the cotyloid cavities 

 being directed inwards so as to throw the soles 

 of the feet very near each other, the base of 

 support becomes very much contracted, and 

 the animal stands very unsteadily. Of all the 

 Quadrumana, the trunk of the Chimpanzee is 

 directed nearest the vertical ; in walking as 

 well as in conformation he approaches nearest 

 to the figure and gait of man. In the Hylo- 

 bates and Laniscus the arms are of sufficient 

 length to enable them to touch the plane of 

 motion with the fingers and assist them in 

 walking; this is resorted to whenever the 

 centre of gravity falls anterior to the base of 

 support formed by the hind extremities. 



The Lemurs are more decidedly quadrupeds ; 

 the large arc described by the spine, directed 

 with its concavity downwards ; the lengthened 

 horizontal direction of the face to the direc- 

 tion of the scapular and cotyloid articulations, 

 contribute to the prone position of the Lemurs. 

 Some of the tribe, as the Lemur tardigradus, 

 are denominated Sloths from their proverbially 

 slow progression. Although the Ourangs have 

 the advantage over other Quadrumana, of walk- 

 ing erect, and having the hands and arms free 

 for prehension and great variety of action, such, 

 as using a club for defence or assault ; yet the 

 Gibbons outstrip the Ourangs in the velocity 

 of their progression, and in their power of 

 swinging and projecting themselves from tree 

 to tree with extraordinary velocity. According 

 to Duvaucel, the Hylobates agilis can launch 

 itself from bough to bough at the distance of 

 forty feet, asunder, apparently without effort or 

 fatigue. Martin relates, that " a live bird being 

 set at liberty in the presence of a female Hylo- 

 bates agilis, she marked its flight, made a 

 long swing to a distant branch, caught the 

 bird with one hand in her passage, and at- 

 tained the branch with her other hand, her 

 aim both at the bird and the branch being as 

 successful as if one object only had gained 

 her attention." The addition of a long and 

 flexible tail in the Cercopithecus, Semno- 

 pithecus, and several other species, gives a fifth 

 organ of prehension which is employed to as- 

 sist them in a variety of motions. The walk, 

 trot, and gallop of the Quadrumana are per- 

 formed upon their four extremities on the same 

 principles as those of quadrupeds in general, 

 but as plantigrade bipeds their locomotion is 

 accomplished like that of man. 



SECT. V. Man. The locomotion of man 

 is that of a plantigrade biped. When the 



* System Anat. dcs Aniinaux. 



body is erect and the face inclined at a small 

 angle above the horizontal plane, the head is 

 exactly balanced on the atlas, and its weight is 

 transferred to the latter with the least expen- 

 diture of muscular action : its axis of motion 

 is bisected by a vertical line passing through its 

 centre of gravity when it is in equilibrio upon 

 the atlas : in every other position of the head 

 the expenditure of muscular action is increased. 

 It has been observed by Daubenton, that, when 

 in the position of a quadruped, man is obliged 

 to elevate the head above the axis of the ver- 

 tebral column, in order to see directly for- 

 wards; and in depressing the head to the 

 earth, the position of the occipito-atlantal arti- 

 culation prevents the jaws reaching the ground. 

 The movements of the head on the atlas are 

 restricted to one plane, namely, the vertical ; 

 but its articulation with the dentata permits 

 a horizontal motion through a large arc of a 

 circle ; the head cannot turn without the atlas, 

 nor the atlas without the head ; on the con- 

 trary, the atlas and dentata revolve upon each 

 other in opposite directions. By means of 

 these two joints, whose axes of motion are at 

 right angles to each other, the head enjoys a 

 considerable range both in the vertical and 

 horizontal planes. 



The vertebral column. The office of this 

 complex structure is purely mechanical. The 

 flexibility of the spine is due to the twenty-four 

 joints which divide its length, and the interposi- 

 tion of the elastic, intervertebral, fibrous tissues. 

 It is upon the elasticity and quantity of the latter 

 that the flexibility of the vertebral column de- 

 pends. The mean proportions of the heights of 

 the cervical, dorsal, and lumbar intervertebral 

 elastic tissues to that of the bodies of the ver- 

 tebrae are estimated by Weber as follows : 



Mil. 



Heights of cervical vertebrae .... 95.85 

 Dorsal ____ 242.95 

 Lumbar .... 135.95 



M.M. 



Heights of intervertebral tissues. . 



Cervical .. 20.70 



Dorsal .. 34.90 



Lumbar . . 42.85 



and their mean diameters, 



MM. 



15. 



25.3 



28.0 



And, admitting their breadth to be equal to 

 their thickness, which is near the truth, their 

 tranverse sections will be as the squares of the 

 breadths, or as 225, 640, 784. Hence, if the 

 cervical, dorsal, and lumbar portions are curved 

 with equal force, their angles of flexion due to 

 their elasticity will be in the following pro- 

 portions : 



. 



2257 \640 



846 : 297 



784 

 298 



that is, the angle of flexion of the dorsal and 

 lumbar portions, notwithstanding their unequal 

 lengths, are nearly equal, whilst the angle of 

 the cervical portion, though of much less 



