The mechanical forces act within the living being according to the same laws as they do in the external world: the chemical powers too, which are the cause of digestion, heat, and respiration, follow the same laws of definite and quantitative proportion as they do in inert matter; but neither the mechanical forces, nor the physical powers, could create a germ; nor could they even awaken its dormant state to living energy, unless a vital power existed in it, the origin of which is beyond the reach of man.
Animals are endowed with nerve-force, in addition to mechanical force and the physical powers which are common to them and vegetables; a force which constitutes their prime distinction, which is superior to all the other powers from its immediate connection with mind, and which becomes more evident, and more evidently under the control of the animal, in proportion as the animal approaches the higher grades of life, and only attains its perfect development in the human race.
The bones of man and the higher animals are clothed with a system of muscles, so attached that the head, eyes, limbs, &c., can be moved in various directions. In each of these muscles the fibres of two sets of nerves ramify, namely, the sensory and the motor nerves.
The sensory nerves convey external impressions to the brain, and by them alone the mind is rendered conscious of external objects. The impressions made by light and sound upon the eye and the ear, or by mechanical touch on the body, are conveyed by the sensory nerves to the brain, where they are perceived, though the impressions take place at a distance from it. Conversely, the mind or will acts through the brain on the motor nerves, which by alternately contracting, relaxing, and directing the muscles, produces muscular motion. Thus the motor nerves convey the emotions of the mind to the external world, and the sensory nerves convey the impressions made by the external world to the mind. By these admirable discoveries, Sir Charles Bell has proved that ‘we are placed between two worlds, the invisible and the material;’ our nervous system is the bond of connection. The connection, however, between the mind and the brain is unknown: it has never been explained, and is probably inexplicable; yet it is evident that the mind or will, though immaterial, manifests itself by acting on matter; that is, as a power which stimulates the nerves, the nerve-force acting on the muscles. Mental excitement calls forth the most powerful muscular strength, and an iron will can resist the greatest nervous excitement. The nervous and muscular forces are perpetually called into action, because, for distinct perception, the muscles require to be adjusted. Mind is passive as well as active: we may see an object without perceiving it, and we may hear a sound without attending to it. We must look in order to see, listen in order to hear, and handle in order to feel; that is, we must adjust the muscular apparatus of all our senses, of our eyes, ears, &c., if we would have a distinct perception of external exciting objects: and that is accomplished by the power of mind acting upon matter.