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3i6 DESIGN IN NATURE<br />
Elasticity can only come into play when the substance which exhibits it is put upon the stretch or is crushed<br />
up, and this stretching out and crushing up in plant and animal movements is, in every instance, due to a vital<br />
operation. Elasticity cannot act of itself, and when it is made to act, it acts by recoil and produces a jerky movement<br />
; but (and this is the curious thing) when jerky movements are witnessed in plants and ammals they are<br />
caused not by elasticity, but by hfe ; for example, by the sudden contracting of living cells and hollow viscera,<br />
with a view generally to the propulsion of fluids, and by the energetic contraction of muscles, with a view to pro-<br />
ducing movements in the travelling organs or other parts of the body. The sudden movements referred to are<br />
vital movements ; movements to a given end and for a purpose. Elasticity, when evoked by life, is always<br />
under control, and it is a mistake to assign to it anything but the most subordinate rols in plant and animal<br />
physics. One has only to watch the movements of protoplasm, of the white blood-corpuscles, of the amoeba, the<br />
sarcous elements of muscles, &c., to be convinced of this. It is an error to suppose that vital movements are only<br />
manifested by contractions, and by elasticity as a counter-acting force. The vital movements are numerous and<br />
various : pushing or elongating movements, pulhng or shortening movements, contracting or closing movements,<br />
expanding or opening movements, spiral movements, &c. The pushing, elongating, opening movements are centri-<br />
fugal in their nature ; the shortening, contracting, closing movements being centripetal. The terms centrifugal<br />
and centripetal are employed in relation to the central portions of the substance exhibiting the movements. The<br />
movements in the hving mass, as a rule, spread from and return to a given point ; that point, for the most part,<br />
corresponding with the position of rest. That hving protoplasm can advance and retire as apart from contraction<br />
and its supposed counter-acting force of elasticity, is proved by the movements of chmbing, sensitive, and insectivorous<br />
plants ; by the advance of the plasmodium of Badhamia vtricularis ; by the amoebic movements of the white blood-<br />
corpuscles ; by the movements of the amoeba, and by the streaming of the pseudopodia of Gromia, and the<br />
Foraminifera generally. The plasmodium of Badhamia vtricularis when feeding advances as a wedge-shaped mass,<br />
no trace of contraction being anywhere observable. The white blood- corpuscles when forcing themselves through<br />
the walls of the capillary blood-vessels, as I have satisfied myself by careful microscopic examination, invariably<br />
throw out a knuckle or wedge of their substance, the thin end of which perforates the vessels. When the<br />
corpuscles have in part forced a passage, they draw or pull the remainder of their substance through the capillary<br />
walls, where they assume their original shape. Similarly, the amoeba invariably moves by projecting one or more<br />
wedge-shaped portions of its substance—the substance streaming towards the apex of the moving wedge or<br />
wedges. The amoeba extends its substance in the direction of travel, and, in a sense, pushes itself forward by<br />
a centrifugal movement. There is no constriction in the advancing mass to indicate contraction of any kind,<br />
and no trace of elasticity can be detected. The advancing mass deliberately—that is, voluntarily—thrusts itself<br />
forward in an aggressive manner. The white blood- corpuscles and the amoeba exhibit both centrifugal and<br />
centripetal movements.<br />
The movements of pseudopodia as seen in Gromia are also centrifugal and centripetal in character ; when Gromia<br />
is searching for food, it extrudes its protoplasm or body substance voluntarily in continuous streams in more or less<br />
straight hnes. When prey is caught the protoplasm is voluntarily and suddenly drawn towards and into the central<br />
mass. The outward centrifugal movement is a vital, non-elastic, advancing, aggressive movement made in search<br />
of food ; the inward, centripetal, retreating, non-aggressive movement (also vital) represents the remaining or com-<br />
plementary half of the capturing act. The centrifugal and centripetal movements are co-ordinated, complemental,<br />
and voluntary, and neither movement would be of any use by itself. The movements of the sarcode of Gromia<br />
(and of the amoeba) are in all respects analogous to the movements of the white blood-corpuscles and the sarcous<br />
elements of muscle, voluntary and involuntary. If a fibrilla of a hving, striated, voluntary muscle be examined<br />
under the microscope it will be seen that the more or less square, cube-shaped sarcous elements forming it are<br />
endowed with a double power, whereby they can elongate first in one direction, and then in another and opposite<br />
direction. When all the sarcous elements of the fibrilla elongate in the direction of the length of the fibriUa, the<br />
fibrilla is said to relax, in which case it increases in length ; when all the sarcous elements elongate transversely<br />
to the length of the fibrilla, the fibrilla is said to contract, in which case it decreases in length. The terms relaxa-<br />
tion and contraction, when so employed, are at once inaccurate and misleading. Contraction, strictly speaking,<br />
implies a diminution in bulk which does not occur when the fibrilla shortens. The sarcous elements are at present<br />
(and contrary to observation) accredited with only one power, namely, the power of contracting or shortening the<br />
fibrilla in the direction of its length ; the power of elongating the fibrilla in the direction of its length being denied.<br />
The sarcous elements, according to the mechanical school, are invested with a centripetal, shortening, or closing<br />
power, but are said to have no centrifugal, elongating, or opening power. They aver that the closing or contracting<br />
power is traceable to the operation of hfe, but that the dilating or elongating power can only be ascribed to elas-<br />
ticity. This theory of muscular action is ahke at variance with the appearances seen under the microscope, and