18. When certain muscles are deprived of this power of contracting, they are said to be paralyzed, or the limb is called paralytic. When they have it in excess, they are in a state of spasm, or convulsion. We see such a state often brought on by excessive drinking. There is no cure for this but by entirely breaking off the habit.

19. The other kind of contractility belongs to every part of the body. It does not depend for its existence on the brain, nor is it at all under the influence of the will, or accompanied with consciousness. Thus the heart and the stomach contract constantly under the application of their proper stimuli, but the brain is not conscious of it; their action is entirely beyond its jurisdiction. This form of contractility has been called insensible.

20. It is owing to the insensible, organic contractility, that the blood circulates in the capillary vessels ; the lymph and the chyle in the absorbents and lacteals, and all the secreted fluids through the vessels that prepare them. In all animals destitute of a heart, the fluids can only be moved by this insensible contractile power. A similar force is supposed to exist in the vessels of plants.

21. These two kinds of contractility, viz., the sensible and insensible, have been compared to the hour and minute hands on the dial of a clock, which are both moved by the same power ; yet the motion of one is insensible to the eye, while that of the other is distinctly visible.

22. By means of the alterative powers, all the changes which take place in the composition of the solids and fluids of the body are effected. By these, the food is changed into chyme, and then into chyle ; chyle into blood, and blood into bone, muscle, cartilage, etc. By these, animal heat is pre served uniform ; and the solids preserve their cohesion, and the fluids their fluidity. In short, it is these powers that for a time successfully resist the agency of chemical laws.

23. Some physiologists attempt to account for every thing that takes place in the body, on chemical or mechanical principles. But the vital laws, or organic forces, form compounds which could never be produced by chemical affinity. In fact, they are antagonistic forces, fighting against each other. The chemist can decompose blood, bile, saliva, albumen, gelatine, and fibrin, but he can not re form one of them. He can no more make a piece of bone, than he can make a diamond.

24. The physical properties of the animal tissues are usually reckoned as five in number, viz. elasticity, extensibility, flexibility, imbibition, and evaporation.

25. Elasticity is one of the physical properties of animal matter. It is that power which tends to restore parts that have been stretched or extended to their former state. It is possessed in the greatest degree by the cellular tissue, which enters largely into the composition of all the structures in the body. All the organs and membranes of the body are in a constant state of extension. All the hollow organs, as the stomach, gall bladder, and blood vessels are kept distended by the volume of their contents. The extensor and flexor muscles, when in a state of inaction, are in a state of extension. If such were not the case, all the organs would contract and shrink to a comparatively small size.

26. When the stomach is empty, its sides contract till they almost touch each other. When a muscle is cut, the wound gapes open, owing to the two parts receding. The cartilages of the ribs are highly elastic, and this facilitates much the function of respiration. The same is true of the substance of the lungs themselves. The cartilages between the bones and the spine are highly elastic. The loss of this elasticity makes a difference of an inch or more between the height of a man in the morning and at evening. It is not unusual for a very tall person to lose an inch in height by dancing all the evening. During sleep, the force of elasticity restores these cartilages to their usual dimensions.

27. The elasticity of the arteries contributes much to the circulation of the blood. The blood as it is forced into these vessels, is constantly reacted upon, by their elastic coats, and in this way driven along towards their termination in the capillary vessels. The same power assists in circulating the lymph and chyle in the vessels which contain them. It is the last function that ceases to act, and it is not wholly destroyed even at death.

28. Flexibility and extensibility are properties existing in various degrees in different parts of the body. The ligaments, or little bands, which tie together the bones, are more flexible than any other part. By observing the astonishing feats of the rope dancer, we see how flexible these parts are. The tendons which connect muscles with the bones they move, are capable of little, if any extension. If they stretched, when the muscles to which they belong contract, the limbs would not move, and the moving force would thus be lost.

29. Imbibition, is another power possessed by living animal bodies. It means the act of drinking in, or taking up fluids, which may be in contact with any part. For example, if a certain fluid be placed in contact with an animal tissue, it will penetrate into the latter, as water would into a sponge, and this property is possessed by all the soft tissues to a greater or less extent. All the serous membranes absorb with great facility. The epidermis or cuticle of the skin permits fluids to pass with difficulty. That water is taken up in considerable quantity, however, is proved by the fact, which has often been proved by experiment, that a man increases in weight by remaining for a considerable time in a warm bath.

30. The following experiments prove the nature of this process. If we fill the intestine of a chicken with milk, and place it in water, we shall see the milk pass through its coats into the water, and the water will pass through in an opposite direction to supply its place. In the same manner, if a bladder be filled with hydrogen gas, and suspended in the air, in a short time it will be found to be mixed with atmospheric air, which has passed through its coats. The result of all the experiments on this subject seems to show, that when any cavity containing a fluid is immersed in another fluid less dense than the former, there is a tendency in the membrane to expel the denser, and absorb the thinner fluid.