CENTRAL NERVOUS SYSTEM. 657 



according to similar anatomical restrictions imposed by the arrangement of the 

 efferent cells, and along the efferent pathway they are directed by isolated 

 fibres either to the voluntary muscles, or by means of other fibres to the 

 ganglia of the sympathetic. In this latter subdivision the arrangement is 

 for diffusion from the proximal to the distal members of the series, and here 

 the area of tissue finally affected is large as compared with the part of the 

 efferent system from which the outgoing impulse may have started. Yet 

 the point at which the most significant diffusion of the impulses occurs is the 

 central system. 



The afferent elements being single cells only, the amount of diffusion which 

 may occur is limited to the branches of this one group of elements alone. 

 The efferent subdivision of the nervous system, so far as it connects with 

 skeletal muscles, represents a single element, but so far as it is connected with 

 the sympathetic system there are at least two elements arranged in series. The 

 arrangement of the central system, however, is but an elaboration of this latter 

 in so far as the number of elements involved may be increased above two. 

 Any incoming impulse entering the central system at any point tends to be 

 diffused over a large portion of the central cells and by them to all the 

 efferent elements, but the path between the point of the arriving impulse and 

 that at which the evident discharge originates in the efferent cells is variable. 

 The permeability of the central system is therefore inconstant, and probably 

 this inconstancy depends on the one hand on the ease with which the incoming 

 impulses are transferred to it and from it, as well as the ease with which they 

 pass among the elements constituting this subdivision itself. The chief prob- 

 lem in the physiology of the central system is, therefore, to determine how 

 the nerve-impulses find their way among the central cells and at what point 

 they pass over to the efferent cells so as to cause an evident response. 



D. REFLEX ACTION. 



The simplest and most constant of the co-ordinated reactions of the nerv- 

 ous system are reflex. The term involves the idea that the response is not 

 accompanied by consciousness, and is dependent on anatomical conditions in 

 the central system which are only in a slight degree subject to physiological 

 modifications. This view of reflex activities is in a large measure justified by 

 the facts, but at the same time it must be held subject to many modifications, 

 and it is not possible to make a hard and fast line between reflex and voluntary 

 reactions. 



The principal features of a reflex act may be illustrated by following a 

 typical experiment. 



Typical Reflex Response. If the central nervous system of a frog be 

 severed at the bulb, so as to separate from the spinal cord all of the portions 

 of the central system above it, the animal is for a time in a condition of col- 

 lapse. If, after twelve hours or more, such a frog be suspended by the lip, it 

 will remain motionless, the fore legs extended and the hind limbs pendent, 

 though very slightly flexed. If such a frog were dissected down to the nervous 



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