THE NERVOUS SYSTEM. 467 



falls strongly on the eye, a tickling may be felt in the nose, exciting 

 sneezing. 



A variety of transference, which may be termed radiation of impres- 

 sions, is shown when an impression received by a nervous centre is dif- 

 fused to many other parts in the same centre, and produces sensations 

 extending 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 sensa- 

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

 tingling 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 accom- 

 panied 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 

 secondary 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 communication 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 centrifugal 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 transference, the reflec- 

 tion may take place from a certain limited set of centripetal nerves to a 

 corresponding and related set of centrifugal nerves; as when in conse- 

 quence 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. Eeflex movements, 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 sen- 

 sori-motor. 



(a) For the manifestation of every reflex action, these things are neces- 

 sary: (1), one or more perfect centripetal nerve-fibres, to convey an im- 

 pression; (2), a nervous centre for its reception, and by which it may be 

 reflected; (3), one or more centrifugal nerve-fibres, along which the im- 

 pression may be conducted to (4), the muscular or other tissue by which 



