GENERAL OBSERVATIONS ON THE DISPOSITION AND COMPOSITION OF THE NERVOUS MAT TER, THE NATURE OF NF.RVOUS ACTIONS, AND THE SUBDIVISIONS OF THE NERVOUS SYSTEM.
The nervous matter presents the singular peculiarity that it alone, of all the varied forms of animal texture, is directly influenced by the mental acts of animals. It is that part of the organism through the immediate agency of which mind operates upon body and body upon mind. Through this connexion with the psychical principle of the animal, sensation is produced, and volition is enabled to exercise its influence on muscular organs. And in the whole range of the mysterious phenomena, which the student of nature meets with, there is no thing so inscrutable as the fact that the work ings of the mind 'can disturb and impair the organization of the nervous matter ; or, on the other hand, that the disorganization of the ner vous matter is capable of deranging mental manifestations.
The existence of this remarkable and pe culiar kind of organic matter is limited to the animal kingdom, and is therefore one of the characteristic features of animals as distin guished from plants. It is obviously the pre sence of a psychical agent controlling and di recting certain bodily acts of animals, which has called into existence the particular appa ratus which the nervous matter is employed to form.
In the largest proportion of the animal king dom, the nervous matter is so disposed or arranged as to form a system complete in itself, and distinct from, although connected with, the other textures and organs. This is called the NERVOUS SYSTEM—the deve lopement of which lia.s always a direct relation to the bodily organization and psychical endow ments of the animal.
The nervous matter is accumulated into masses, forming what are denominated CEN• TRES of nervous actions ; and it is also deve loped in the form of fibres, filaments, or mi nute threads, which, when bound together, constitute the Nertves. The latter are in fernuncial in their office; they establish a communication between the nervous cen tres and the various parts of the body, and vice versfi.; they conduct the impulses of the
centres to the periphery, and carry the impres sions made upon the peripheral nervous rami fications to the centres. Nor are the nerves mere passive instruments in the performance of their functions ; but produce their proper effects through their susceptibility to undergo molecu lar change under the influence of appropriate stimuli.
The centres are the great sources of ner vous power ; they are the laboratories in which the nervous force is generated. The mind appears to be more immediately connected with one of them, which, pre-eminent on that account, exerts a certain control or influence over its fellows.
In the centres there are two kinds of ner vous matter, distinguished by certain anato mical characters and by certain physiological properties and uses. The one is globular or vesicular in structure, grey in colour—dynamic as regards office. The other is fibrous, its fibres being tubes containing nervous matter; it is white in colour, and is devoted to act as a conductor of impulses to and from the grey matter. The white matter is that of which the nerves are composed, and the two kinds of matter do not occur together any where but in the nervous centres ; in fact, their 'co-existence in any part of the nervous system is sufficient to constitute that part a centre of nervous action.
In the lowest creatures the existence of ner vous matter is as yet problematical. It is supposed by some physiologists that it is dif fused in a molecular form throughout the body of the animal, and the muscular tissue being likewise disposed in a similar way, the one may act upon the other at every point. Were this supposition true, it might be further con jectured that, under such circumstances, only one kind of nervous matter, thedynamic, would exist ; for as the office of the white nervous matter is chiefly to propagate or conduct to distant parts the changes which originate in the grey matter, the former would not be re quired in animals, in which the elements of the grey matter are in contact with those of the other textures at every part of the body.