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Of osseous tissue, hardness, stiffness, and toughness. 2. Of muscular tissue, contractility and irritability. 3. Of nervous tissue, irritability and conductivity. 4. Of cartilaginous tissue, stiffness and elasticity. 5. Of connective tissue, toughness and pliability. 6. Of epithelial tissue, ability to resist the action of external forces and power to secrete.
Again, in the unmodified ectoderm, as we see it in the Hydra, the units are all endowed both with impressibility and contractility; but as we ascend to higher types of organization, the ectoderm differentiates into classes of units which divide those two functions between them: some, becoming exclusively impressible, cease to be contractile; while some, becoming exclusively contractile, cease to be impressible.
I am not now alluding to such phænomena, at once rare and conspicuous, as those exhibited by the leaflets of the sensitive plants, or the stamens of the barberry, but to much more widely spread, and at the same time, more subtle and hidden, manifestations of vegetable contractility.
Let us in imagination simplify Driesch's experiment, for the sake of gaining a clearer view of its meaning. In a certain embryo at an early stage are certain cells whose descendants should form the lining of the intestine and be used in the adult for digestion. A second set of cells should form muscle endowed mainly with contractility.
The progress of physiology has further shown, that the contractility of the muscles and the irritability of the nerves are purely the results of the molecular mechanism of those organs; and that the regular movements of the respiratory, alimentary, and other internal organs are governed and guided, as mechanically, by their appropriate nervous centres.
Quite fairly it may be urged that the writer of passages like these would, if writing in modern language, and with the aid of modern conceptions, have expressed himself much as Professor Huxley does when, declaring that the circulation of the blood and the regular movements of the respiratory, alimentary, and other internal organs are simply 'affairs of mechanism, resulting from the structure and arrangement' of the bodily organs concerned, from 'the contractility of those organs, and from the regulation of that contractility by an automatically acting nervous apparatus; that muscular contractility and the automatic activity or irritability of the nerves are 'purely the results of molecular mechanism; and that 'the modes of motion which constitute the physical bases of light, sound, and heat are transmuted by the sensory organs into affections of nervous matter, which affections become 'a kind of physical ideas constituting a physical memory, and may be combined in a manner answering to association and imagination, or may give rise to muscular contractions in those reflex actions which are the mechanical representatives of volition. Quite fairly may a doctrine, capable of being thus translated, be described as leading 'straight to materialism. Quite justly may its author be claimed by Huxley as joint professor of a materialistic creed.
Beyond all question, the circulation of the blood is simply a matter of mechanism, and results from the structure and arrangement of the parts of the heart and vessels, from the contractility of those organs, and from the regulation of that contractility by an automatically acting nervous apparatus.
And, so far as the conditions of the manifestation of the phaenomena of contractility have yet been studied, they are the same for the plant as for the animal. Heat and electric shocks influence both, and in the same way, though it may be in different degrees.
This medicine possesses a very great power over the uterus, rousing its dormant or debilitated contractility, and stimulating it to an extra performance of this necessary function after its natural energy has been in some measure destroyed by forcible but useless action.
Many forms are motile some in virtue of fine thread-like flagella, and others through contractility of the protoplasm. The great majority multiply by simple fission, each parent cell giving rise to two daughter cells, and this process goes on with extraordinary rapidity. Other varieties, particularly bacilli, are propagated by the formation of spores.
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