THE NERVOUS SYSTEM. 85 



tending far beyond the part from which the primary impression was 

 derived. Hence, as in the former cases, result various kinds of what have 

 been denominated sympathetic sensations. Sometimes such sensations 

 are referred to almost every part of the body: as in the shock and ting- 

 ling of the skin produced by some startling noise. Sometimes only the 

 parts immediately surrounding the point first irritated participate in the 

 effects of the irritation; thus the aching of a tooth may be accompanied 

 by pain in the adjoining teeth, and in all the surrounding parts of the 

 face; the explanation of such a case being, that the irritation conveyed 

 to the brain by the nerve-fibres of the diseased tooth is radiated to the 

 central ends of adjoining fibres, and that the mind perceives this second- 

 ary impression as if it were derived from the peripheral ends of the fibres. 



3. Reflection. In the cases of transference of nerve-force just 

 described, it has been said that all that need be assumed is a communica- 

 tion of the excited condition of an afferent nerve to other parts of its 

 nerve-centre than that from which it takes its origin. In the case of 

 reflection, on the other hand, the stimulus having been conveyed to a 

 nerve-centre by a centripetal nerve, is conducted away again by a cen- 

 trifugal nerve, and effects some change motor, secretory or nutritive, at 

 the peripheral extremity of the latter the difference in effect depending 

 on the variety of centrifugal nerve secondarily affected. As in transfer- 

 ence, the reflection may take place from a certain limited set of cen- 

 tripetal nerves to a corresponding and related set of centrifugal nerves; 

 as when in consequence of the impression of light on the retina, the iris 

 contracts, but no other muscle moves. Or the reflection may extend to 

 widely different parts: as when an irritation in the larynx brings all the 

 muscles engaged in expiration into coincident movement. Reflex move- 

 ments, occurring quite independently of sensation, are generally called 

 excito-motor ; those which are guided or accompanied by sensation, but 

 not to the extent of a distinct perception or intellectual process are termed 

 sensori-motor. 



Laws of Reflex Action. (a) For the manifestation of every reflex 

 action, these things are necessary: (1), one or more perfect centripetal 

 nerve-fibres, to convey an impression; (2), a nervous centre for its recep- 

 tion, and by which it may be reflected; (3), one or more centrifugal 

 nerve-fibres, along which the impression may be conducted to (4), the 

 muscular or other tissue by which the effect is manifested (p. 80, Vol. 

 II.). In the absence of any one of these conditions, a proper reflex action 

 could not take place; and whenever, for example, impressions made by 

 external stimuli on sensory nerves give rise, to motions, these are never 

 the result of the direct reaction of the sensory and motor fibres of the 

 nerves on each other; in all such cases the impression is conveyed by the 

 afferent fibres to a nerve-centre, and is therein communicated to the 

 motor fibres. 



