CHAPTER XXIX. THE PHYSIOLOGY OF MIND. Differentiation into and of life. Contractility, the fundamental character of the latter. IN N the chapter on the Direction of Development it has been stated that the fundamental differentiation in organs of animal development is that into the systems of organs vegetative which belong respectively to the vegetative and the animal animal life. It is necessary for me to state over again part of what I have already stated in that chapter, though I now approach the subject from a different point of view. The organs of vegetative life essentially consist of an apparatus for transforming matter; those of animal life essentially consist of an apparatus for transforming energy. The fundamental attribute of the apparatus of animal life is contractility; that is to say, the characteristic power of muscular tissue to contract under the influence of a stimulus, and in contracting to transform energy from the vital into the motor form :1 in other words, to do work by contracting. In those simple animals which have muscles but no nerves, as especially the Hydrozoa, the stimulus under which the muscles contract consists in the contact of food or of some other foreign substance; and such a stimulus, if it is applied to one part of the animal's muscular tissue, will slowly propagate itself; so that, if one tentacle is irritated, the other tentacles will also contract. But in those more highly organized animals which have a complex nervous system, the stimulus under which the muscles contract usually consists in a flow of nervous energy. The difference between the action of muscles that usually contract under a stimulus applied to themselves, and muscles 1 See the chapter on the Dynamics of Life. that usually contract under a stimulus transmitted through muscular. The nervous system is developed by differentiation out Nervous of the muscular system, and in its first and lowest deve- system developed lopment appears to have no vestige of sensation and no out of other function than that of transmitting stimuli to the Its muscles. The muscular tissue of the Hydrozoa, as stated primary function is above, transmits a stimulus but slowly. Nervous tissue, on to the contrary, transmits it with a rapidity which is practi- stimuli cally instantaneous; and this, of course, makes the motions to the more rapid, and the whole life more energetic, in those animals which have a nervous system than in those which are without one. mit muscles. 1 It is, however, a very remarkable, and at present an inexplicable fact, that the nervous system is not, in any Nervous animal whatever, so simple as might be thought from the system foregoing account of its functions. The nerve-fibres are simple. in no case directly laid on, as it were, like telegraph wires, 1 All the facts and opinions respecting the anatomy and physiology of the nervous system stated in this chapter are taken from Carpenter's Human Physiology, except where I advance any opinion as my own, never for the purpose of transmitting stimuli directly from the skin, or any other part which is more exposed to stimuli than the rest, to the muscles that have to make the response to the stimuli. In the simplest nervous system that appears to be possible under the laws of life, there Ganglia. must be at least two fibres, meeting in a ganglion, and acting one on the other through it. One of the fibres conducts the stimulus from the skin, or wherever its outer extremity is situated, to its inner extremity at the ganglion. The other fibre conducts the stimulus from the ganglion to the muscle in which it terminates, and causes the muscle to contract. This is what is called "reflex action;" the stimulus being, as it were, reflected from the ganglion. Reflex action. As already mentioned, nervous structure is a development and outgrowth of muscular structure, being developed by differentiation from it. This is observed, both in watching the successive stages of the development of the highest animals, and in comparing the various members of the animal kingdom, from the simplest to the most complex development of a nervous system. And it will be obvious from what has been stated above, that nervous function also is a development and outgrowth of muscular function, ated from being developed by differentiation from it. In the function. Hydrozoa, there are no separate organs for the purpose of transmitting stimuli, nor any organs at all which transmit them, except the muscles themselves. But when a nervous system is developed, the function of transmitting stimuli is separated from the ordinary muscular function, and assigned to the nerves. Nervous function differenti muscular Sensation does not exist at first. In the simplest and lowest development of a nervous system, the action of the nerve-fibres on their ganglia is probably unattended by any sensation; and this continues to be true of large parts of the nervous systems even of man and the higher animals. Higher up in the animal scale, sensation appears: the action of some-not all—of the nerve-fibres on their ganglia produces sensation. We cannot tell where it begins. I think it most likely, that sensation begins where organs of special sense come into sense. existence; and as eyes are the most generally distributed It begins probably of these in the animal creation, I think it most likely that with sensation is nearly co-extensive with the possession of special eyes. But this is incapable of proof: we have no criterion whatever of its presence or absence. Sensation is in itself, of course, perfectly inscrutable. It is utterly impossible that we can ever know how or why it is that the flow of a current of a peculiar kind of energy along a nerve to its ganglion should be accompanied by sensation. But we might have expected to find sensation the peculiar function of some particular kind of tissue, so that the presence of sensation might be inferred with certainty from its presence, and the absence of sensation from its absence. Such, however, is not the case. Some nerves and ganglia Sentient are sensory, others are not so; and the microscope, so far sentient as we yet know, shows no difference whatever between nerves the structure of the two. and in histologically alike. develop and of We thus see that the sensory function is not a primary Parallel or fundamental endowment of the nervous system, but has ment of been added to its original functions in the course of deve- organs lopment. The history of the development of the nervous functions. organs is parallel with the history of the development of the nervous functions. The spinal cord, which is the principal nervous organ of the insentient life, is developed first in the embryo, and the sensory ganglia grow out of it. The sensory ganglia are situated within the skull, but are distinct from the cerebrum, or true brain. Besides the ganglia of the special senses, there is among them a pair of ganglia called the thalami optici, which (notwithstanding their name) are believed to be the nervous centre for the sense of touch. In close proximity to the sensory ganglia is another pair of ganglia called the corpora striata, which Corpora are believed (though the subject is an obscure one) to be striata; the ganglionic centres for the nerves of motion, in so far as 1 In order to guard against a probable misconception, it ought to be stated that, so far as we know, the ganglia are not the seats of sensation any more than the nerve-fibres. Sensation is produced when certain ganglia receive a stimulus from their nerve-fibres; but the ganglion is insentient if acted on by pricking or in any other way except through its fibres, and the fibres are insentient if they are cut off from their ganglia. motion is not merely reflex, but determined by sensation their rela- and will. The relation of these to the nerves and ganglia tion to the of sense is as follows:-An impression of sense is transganglia. mitted by some of the nerve-fibres of that sense to its sensory Consensual action: its similarity to merely reflex action. Sensation only the guide to action. ganglia, and the reception of the impression by the ganglia produces the sensation which is appropriate to that impression: the sensory ganglia, in their turn, act on the corpora striata, which are motor ganglia; and the latter send down the motor nerves whatever motor impulse is necessary in order to make the appropriate response to the sensation. For instance a flash of light falls on the retina, and the impression is telegraphed by the optic nerve to the optic ganglia, where it produces the sensation of light; the optic ganglia act on the corpora striata, and cause them to send a motor impulse to the muscles of the eyelids, which closes the eyes, and thus makes the appropriate response to the impression of the flash of light. This is what Dr. Carpenter calls consensual action. The chain of cause and effect is exactly the same in consensual action as in merely reflex, except that in consensual action one of the links of the chain is sentient, and the motor action will not be produced unless sensation is felt. Both reflex and consensual action consist simply in, first, an impression sent from the exterior extremity of a nerve to the nervous centre; and, second, a motor impulse sent outwards from the nervous centre in response to the impression. It will be observed that in the foregoing account of at first is consensual action, sensation is described as existing, not by itself, but only as the intermediate link between impressions received by the organism from without, and the muscular actions that constitute the appropriate response to those impressions. Sensation existing by itself, and not necessarily leading to action, appears to belong to a higher development of life, and to be the preparation for Mind. An insect may no doubt have sensations without making any motion in response to them: if such sensations exist, we can have no evidence of their existence. But we know that the first and lowest functions of the nervous system |