G07 



moves, by a viscid and glutinous fluid which coagulates and forfcis on his 

 track, a shining varnish. This creature fixes itself, likewise, on the 

 ground, by forming a vacuum with the part of his body on which it 

 crawls, which is broad, fringed, and well adapted to answer the purpose 

 of a cupping-glass. It is by this double resource of a viscid and glu- 

 tinous fluid, and of a contractile exhauster, that the snail fixes the fore 

 part of his body, and then draws towards this fixed part, the rest of his 

 body loaded with the shell. This part of the snail, by which it fastens 

 itself to the ground on which it crawls, bears some analogy to the tenta- 

 cula which assists the progression of the sepia and other cephalopodous 

 rnolusca. 



CXCIII. Partial motions performed by the upper extremities. These 

 motions will furnish us additional illustrations of the elastic curve, or of 

 the third lever, to the theory of which, may be be referred almost all the 

 motions of man and of the lower animals. This idea simplifies and 

 facilitates, in a remarkable manner, the study of animal mechanics; it 

 may be considered as a general formula, by the help of which we may 

 obtain a solution of all the problems of this interesting part of physical 

 science. Its application particularly distinguishes what has just been 

 stated on motion, from what had been, heretofore, written on the same 

 subject. 



The upper extremities, in man, are not employed in motions of pro- 

 gression, at least, not generally, except in a few instances, as for ex- 

 ample, when the limbs being extended and the hand having a firm hold 

 of a body, the action of the great pectorals draws the whole body, lying 

 prone on a horizontal surface, or suspended. 



We experience a difficulty in climbing, because our hands alone enable 

 us to grasp the body, on which this mode of progression is to be effect- 

 ed, while the four extremities of the quadrumana and the sharp claws of 

 cats, those of climbing birds, render this action easy and natural to all 

 these animals. 



There exists so great a disproportion, in point of length and strength, 

 between our upper and lower extremities, that walking on all fours can 

 never be natural to the human species; besides, as Daubenton observed, 

 the situation of the foramen magnum of the occipital bone, in man, renders 

 this attitude exceedingly uneasy. Its situation, near the centre of the base 

 of the skull, and nearly horizontal, prevents the head from being- raised 

 sufficiently high to enable us to turn our face forward and to see before us, 

 and if we bring the head downward, it strikes the ground wjth its sum- 

 mit, or with the forehead*. But our upper or thoracic limbs, though of 

 no use in conveying us whither our wants require, are almost exclusively 

 destined to perform motions by which we act on the objects towards 

 which we have brought ourselves. 



If we wish to push, or to draw towards us, or to propel afar a moveable 

 body, to compress, to elevate, or to lower it, our upper extremities are 

 almost exclusively engaged in this office. 



In pushing, man places himself between the obstacle and the ground ; 

 he bends his body between these two points, by bringing all his limbs in- 

 to a state of flexion : he then extends them; his whole body represents a 

 spring which is released and recovers itself, and the two extremities of 



* Dictionnaire d'Histoire Natvirelle do 1'Encyclopedie methodique. Introduction, 

 page 21 et suiv. 



