590 



NERVOUS SYSTEM. (NERVOUS ACTIOKS.) 



glussopharyngeal, which are freely distributed 

 upon the mucous surface of the pharynx. This 

 stimulus causes a change in the medulla oh- 

 longata, in which those nerves are implanted, 

 and that change is propagated thence by the 

 p/iaryngeul branches of the vagus to the mus- 

 cles which contract the pharynx in the act of 

 deglutition. 



In such an action the change in the nerves 

 by which the muscular contraction is excited 

 must take place in a two-fold direction, first, 

 from the circumference to the centre, from the 

 point of application of the stimulus to that at 

 which the nerves become implanted in the 

 centre; and, secondly, from centre to circum- 

 ference, from the point at which the stimulus 

 foils upon the centre to that where the nervous 

 fibres mingle themselves with the muscles of 

 the part to be moved. The stimulus, which is 

 incident upon the nervous centre, is said to be 

 reflected by it to the muscular textures, the 

 immediate agents of the required movement. 

 Hence Dr. Marshall Hall has proposed to dis- 

 tinguish a special system of incident and reflex 

 nerves, or of excito-motory nerves, the former 

 being excitor, the latter motor. This distinc- 

 tion, considered anatomically, is as yet quite 

 hypothetical, for we have no unequivocal proof 

 that the nerves of sensation and volition, which 

 in their ordinary mode of action are afferent 

 and efferent as regards the brain, may not be 

 competent by the relation which they must 

 necessarily bear to the spinal cord, to perform 

 the actions which are thus assigned to a dis- 

 tinct system. The determination of this point 

 depends more upon the solution of certain pro- 

 blems respecting the anatomical constitution 

 of the nervous centres, than upon any purely 

 physiological experiments. It will be made 

 the subject of careful examination at a subse- 

 quent part of this article. 



The great peculiarity of this class of nervous 

 actions is their independence of 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 felt. 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, then, 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 

 spinalis), 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 of animal life, a distinction which, as it 

 was dependent on his untenable hypothesis of 

 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 highly developed, 

 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 which 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 assign to it a peculiar action, 

 different from that of the nerves connected with 

 the brain and spinal cord. Bichat calls it the 

 nervous si/stem of organic life. Previous to his 

 time it was known as the great intercostal nerve 

 (nervus inter costalis), the great sympathetic 

 nerve (nervus si/mpathicus magnus), 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 weganglionic 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 terras sympathetic 

 nerve and ganglionic system are those which 

 appear liable to fewest objections, although by 



