GENEKAL PHYSIOLOGY OF NERVE-CENTRES. 783 



collections of reflex centres. The special characters of reflex action as 

 produced by the conduction of a stimulus through the spinal cord will 

 subsequently receive attention more in detail. At present a general 

 outline of the production of such reflex action is all that is needed. 



If in an animal the cerebrum be removed by section from the 

 spinal cord,— and such an experiment may be best performed on a cold- 

 blooded animal, — stimuli applied to varying parts of the body surface 

 will result in the production of muscular movement. If in a frog the 

 cerebrum be removed by a section forming a tangent to the anterior 

 part of the tympanic membrane, the frog will apparently be in a normal 

 condition, as far as its posture is concerned. If after the shock of the 

 operation has passed away the toe of such a frog be pinched, the animal 

 will jump; or, in other words, conduction of the sensory impulse to the 

 spinal cord is reflected in the complicated co-ordinated movement of 

 jumping. It is evident, therefore, that the sensory impulse is not simply 

 reflected from the nerve-cell, but that, reaching the nerve-cell, it may 

 there be converted into afferent impulses 

 which may be of the most complex character. 



Coughing and sneezing, also, are illustra- 

 tions of this statement, where the slightest 

 mechanical irritation of various parts of the 

 respiratory mucous membrane may produce 

 complex muscular movements which are out 

 of all proportion in their complexity and 

 vigor to the afferent impulses which inaugu- FlG - 331.— scheme of a re- 



° is flex Arc. (Landois.) 



Tate them. While this iS, however, tO a S, skin; M, muscle: N, nerve-cell, with 



of, afferent, aud ef, efferent, fibres. 



certain degree true, within limits the nature 



of the efferent impulse is dependent upon the nature of the afferent 



impulse. 



If after removing the cerebrum from a frog the flank of the animal 

 be gently stroked, muscular movement will occur simply as feeble 

 twitching of the muscles at the point of stimulation. If the stimulus 

 be increased in intensity the neighboring muscles are also implicated, 

 and a still further increase in severity in irritation ma3 r lead to the 

 implication of nearly all the muscles of the bod}\ 



A connection may also be recognized between the locality of stimu- 

 lation and the nature of the resulting movement; thus, stimulation of 

 the larynx will invariably cause coughing ; of the mucous membrane of 

 the nostril, sneezing ; of the mucous membrane of the eye, weeping and 

 lachrymation, or, to go back to the lower animals, reflex action following 

 from a stimulus applied to the skin of the brainless frog will be so 

 adapted as to remove the irritating body. Thus, if a scrap of paper 

 moistened with acid be placed on the right flank of such a frog, the 



