Problems of Life and Mind. Second series - George Henry Lewes (thriller books to read txt) 📗
- Author: George Henry Lewes
- Performer: -
Book online «Problems of Life and Mind. Second series - George Henry Lewes (thriller books to read txt) 📗». Author George Henry Lewes
While it is thus conceivable that all organisms may resemble each other, and all differ, owing to the similarities and diversities in the “conditions of existence” (and among those conditions that of descent is of wide range), it is not very readily conceivable how advantage in the external struggle could have determined the varieties of form and function, because many differentiations give no superiority in the struggle. As Mr. St. George Mivart urges, “Natural Selection utterly fails to account for the conservation and development of the minute and rudimentary beginnings, the slight and infinitesimal commencements of structures, however useful those structures may afterwards become.”78 And this is undeniable on the supposition that Natural Selection is an agency not identical with the variations of growth, but exclusively confined to the accumulation of favorable variations.
141. In estimating the two hypotheses—First, of Descent from one primordial germ, and the modifications due to Natural Selection, or, as I should say, expressed in Selection; and Secondly, of Descent from innumerable germs having initial differences, which differences radiated into the marked modifications, there is this superiority to be claimed for the first, that it is more easily handled as an aid to research, and is therefore more decidedly useful. The laws of Organic Affinity are at present too obscure for any successful application. I only wish to point out that the theory of Descent is an imaginary construction of what may have been the process of species-formation, not a transcription of the process observed. It constructs an imaginary Type as progenitor of a long line of widely different descendants. The annelid which is taken as the ancestor of the vertebrates is not any annelid known either to zoölogists or geologists, but a generalized and imaginary type. So daringly liberal is the imagination in endowing the ancestor with whatever may be required for the descendants, that Mr. Darwin thinks it probable, from what we know of the embryos of vertebrates, that these animals “are the modified descendants of some ancient progenitor which was furnished in its adult state with branchiæ, a swim-bladder, four simple limbs, and a long tail, all fitted for an organic life,” (p. 533); and Dr. Dohrn conceives the original type to have contained within itself all that has been subsequently evolved in the highest vertebrate, the other and less elaborate organisms being mere degradations from this type.79 This use of the imagination, although not without advantages, is also not without dangers. It may direct research, it must not be suffered to replace research.
THE NERVOUS MECHANISM.
“All the functions of the nervous system are as dependent upon its structure and nature, as the accurate indication of time upon the construction of the chronometer.”—Prochaska.
“Unser Wissen wird nie vollendet, ist und bleibt Stückwerk; dessen Ergänzung das Streben und Hoffen der forschenden Denker bleiben wird für alle Zeit.”—Radenhausen, Osiris.
Can spin an insubstantial universe
Suiting our mood, and call it possible,
Sooner than see one grain with eye exact,
And give strict record of it.”
“If we compare the teachings of our books with what Nature is constantly showing, we find there is no agreement between those two sources of learning.”—Brown Séquard.
SURVEY OF THE SYSTEM.
1. Our knowledge of mental processes is derived from reflection on our personal experiences, combined with inferences from our observation of other men and animals, under similar conditions. The processes are complex and variable; so complex and variable, that knowledge of their component factors can only be gained through long tentative study, aided by fortunate circumstances which present these factors separately, or at any rate in such marked predominance as to fix attention. This subjective analysis of the processes has to be supplemented by, and confirmed by an objective analysis of, the conditions, external and internal: the facts of Feeling have to be traced to facts of Physiology, which will exhibit that Physical Basis of Mind so earnestly sought by the inquirer.
Both the subjective and the objective analysis are at present in a very imperfect state. Although there is much confident assertion and “false persuasion of knowledge” in both regions, there is, unhappily, little that can be seriously accepted as demonstrated. In the present volume we shall concern ourselves almost exclusively with the objective analysis, and do our utmost to mark what is mere inference from what is verified observation. It is only by Observation that facts can be settled; however Analogy and Inference may suggest where the truth may lie, they are finger-posts, not goals. At the best they only tell us what Observation would reveal could the processes be submitted to Sense.
In a loose and general way every one knows that the Nervous System is a dominant agent in all sentient processes; although not by any means the only agent, yet, because of its predominance, it is artificially accepted as the only one. With the greater complexity of this system, there is observed a corresponding increase in the variety of sentient phenomena. The labors of anatomists have secured a tolerably exact plan of the topographical distribution of this system; a somewhat chaotic mass of observation and inference passes as a description of its elementary structure. The labors of physiologists have succeeded to a small extent in localizing certain functions in certain organs of this system. But imperfect as our knowledge of the elementary structures is, our knowledge of the functions is still more so. I wish I could say otherwise, and that I could ask my readers to accept with confidence what teachers confidently propound. The attitude of scepticism is always repulsive; the sceptic is seldom received without disfavor, because he throws on us the labor of investigation there where we wish for the confidence of knowledge. Yet it is only by facing the facts that we can hope one day to solve the great questions.
2. The nervous system has, in our artificial view of it, two divisions: the Peripheral, which connects the organism with the external world; and the Central, which connects each part of the organism with all the other parts. Although the system is constituted by various tissues—neural, connective, vascular, and elastic—it receives its characteristic designation from nerve-fibrils, nerve-fibres, and nerve-cells; just as the muscular system receives its designation from contractile cells and fibres. This neural tissue assumes three well-marked forms: 1°, nerves, which are bundles of fibres and fibrils, enclosed in a membranous sheath; 2°, ganglia, which are clusters of cells, fibres, and fibrils, sometimes enclosed in a sheath, sometimes not; 3°, centres, which are artificial divisions of the neural axis, serving as points of union for different organs.
In the Invertebrata the neural axis is the chain of ganglionic masses running along the ventral side, and giving off the nerves to organs of sense, and to the muscles. It may be seen represented in Fig. 1.
Fig. 1.—Nervous system of a beetle. The small round masses, or ganglia, are seen to be connected by longitudinal fibres, and from the ganglia issue fibres to the limbs, organs of sense, and viscera.
In the Vertebrata the axis is dorsal, and is called the cerebro-spinal axis, including brain and spinal cord. When we look at this structure superficially we see various nerves radiating from it to skin, glands, and muscles; but a closer examination, enlightened by knowledge of function, shows that some of these nerves pass into it from the various surfaces and sense-organs, and are therefore called afferent or sensory; whereas another set passes out of it to glands and muscles, and these nerves are therefore called efferent or motory. There are also fibres which, passing from one part of the great centre to another, are called commissural.
To this brief account of the cerebro-spinal system may be added a word on the connected chain of ganglia and nerves known as the Sympathetic, because it was formerly supposed to be the organ through which the various “sympathies” were effected. It is now held to be the system devoted to the viscera and blood-vessels; but there is still great want of agreement among physiologists as to whether it is an independent system, having a special structure somewhat different from that of the cerebro-spinal, or whether it is simply a great plexus of nerves and ganglia, only topographically distinguishable from the rest of the nervous system. Into this point it is unnecessary for me to enter here. Enough to say, that I entirely agree with Sigmund Mayer in adopting the second view.80 In no histological character, yet specified, are the sympathetic nerves and ganglia demarcated from the others. There are, indeed, more non-medullary fibres (the gray fibres of Remak) in the sympathetic; but the same fibres are also abundant in the cerebro-spinal system; and the sympathetic has also its large medullary fibres.
3. The Centres are composed of two substances: the gray and the white. The gray substance is often called the vesicular because of its abundant cells; but it has even more fibres than cells, and the white substance has also a few cells.81 The gray substance is distributed over the surface of the brain—in the convolutions; and in various other parts of the encephalon. It surrounds the central canal which forms the ventricles of the brain and is continued as a very small cavity all down the spinal cord. Besides entering into the important and conspicuous masses known as the cerebral ganglia—(the optic thalami, and corpora striata)—the gray substance is massed in the corpora quadrigemina, crura cerebri pons varolii, and medulla oblongata. We shall have occasion to refer to each of those parts. Until modern times all the masses included in the skull under the familiar term Brain (or the technical term Encephalon) were regarded as the only centre, and also as the origin of all the nerves. Nor has this notion even yet entirely disappeared, although the spinal cord is known not to be a large nerve trunk, but a centre or connected chain of centres, structurally and functionally similar to the cranial centres. The shadow of the ancient error still obscures interpretation of the part this spinal cord plays in the sentient mechanism; and thus although the cord is universally admitted to be a centre for “sensitive impressions,” it is usually excluded from Sensation. This widespread and misleading notion will be critically examined in a future problem.
4. Beginning our survey of the cerebro-spinal axis with the Spinal Cord, we observe it to consist: 1°, of central gray substance surrounding the scarcely visible canal, which is all that remains of the primitive groove in the germinal membrane (§ 9); 2°, irregular gray masses, called the anterior and
Comments (0)