The great peculiarity of this class of nervous actions is their independence if the mind. An act of the mind forms no necessary part of their mechanism. At the same time there are certain of them which do not take place without the mind being conscious of the change. The act of deglutition above referred to, although quite independent of the mind, does not generally take place without being ft/I. But the change in the pupil, consequent upon the stimulus of light acting through reflex nerves upon the iris, is not at all perceived by the individual, and is therefore in every respect independent of mental change. Let it be remembered, theu, that there are some physical nervous actions which the mind is not conscious of, and others of which the mind will always, or at least generally, take cognizance.
We may conclude this brief reference to nervous actions by the following classification of them:— Psychical or mental nervous actions:— Actions of perception.
Actions of emotion.
Actions of volition.
Physical nervous actions:— Actions from a physical change originating in the nervous centre; as in disease.
Reflex actions— a, with consciousness.
b, without consciousness.
Anatomical subdivision of the nervous sys tem.—The nervous centres, as they are found in the Vertebrate series, are distinguished as the brain, or encephalon, the spinal cord ( medulla spinulis), the ganglions. In the Imertebrata, the centres all bear the anatomical characters of ganglions, although, doubtless, they present some analogy in office to those specially distin guished among Vertebrata. Their arrangement varies considerably according to the differences of form of the various invertebrate classes.
The brain and spinal cord, and the system of nerves connected with them, constitute the cerebro-spinal portion of the nervous system, which Bichat distinguished as the nervous sys tem if animal tile, a distinction which, as it was dependent on his untenable hypothesis y* the two lives, ought now to be discarded. The only subdivision of the nervous system which can be conveniently adopted must rest upon the basis of anatomy. There is not a sufficient distinctness of function in different portions of the nervous system to justify the separation of them on physiological grounds.
There are very numerous ganglions connected with the cerebro-spinal system. These are the ganglions on the posterior roots of spinal nerves, the ganglion of the fifth pair, those of the glosso-pharyngeal, and of the vagus. They are
conveniently distinguished as cerebro-spinal ganglions.
A large portion of the nervous system is made up entirely of ganglions, with their con necting cords and nerves, which ramify in a plexiform manner among various internal viscera and upon the coats of bloodvessels. In the vertebrated animals, where it is highlydeveloped, it is disposed as a chain of ganglia on each side of the spine, and at the base of the skull, near the foramina, through which the spinal and encephalic nerves pass out; and at all these situations it forms a very intimate connexion with the nerves of the brain and spinal cord.
This portion of the nervous system exhibits many peculiarities referable to its composition, its mode of arrangement and its connexion with the organs among wh'ich its nerves ramify, which, at least, entitle it to be considered apart from the cerebro-spinal system ; and many go so far as to affirm its entire independence of that system, and to assi,rn to it a peculiar action, different from that of th°e nerves connected with the brain and spinal cord. Bichat calls it the nervous system of organic life. Previous to his time it was known as the great intercostal nerve (nervus intercostalis), the great sympathetic nerve (nertnis symputhicus mugnris), and now it is very commonly described under the latter name. The term visceral nerve has also been proposed for it. It has also been distinguished as the gan,glionic system. It is difficult to find an unexceptionable name for it which does not involve the adoption of some theory respecting its function. On the whole, the terms sympathetic nerve and ganglionic system are those %vhich appear liable to fewest objections, although hy no means free from them, and they will be employed in the course of this article.
Such are the only subdivisions of the nervous system which anatomy appears to warrant. Others have been proposed; but as they are founded upon physiological opinions which are as yet hypothetical, it is unnecessary to discuss them at present.
NERVE.—(vEvgov, nervus ; Germ. nerve ; Fr. nerf.) The nerves perform the inttrnancial office in the nervous system by maintaining communications between the various organs and tissues and the nervous centres. They are bundles of threads of various size, surrounded by sheaths of membrane, with more or less of areolar tissue interposed.