CENTRAL NERVOUS SYSTEM. 209 



ous system are termed reflex. The term involves the idea thai the response 

 is not accompanied by consciousness, and is dependent on anatomical condi- 

 tions in the central system which are only in a slight degree subject to physi- 

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



justified by the facts, but at the same time it must he 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 : 



If the central nervous system of a frog be severed at the bulb, so as to 

 separate from the spinal cord all the portions of the central system above it, 

 and the brain be destroyed, the animal is for a time in a condition of collapse. 

 If, after recovering from the immediate shock, such a frog be suspended by 

 the lip, it will remain motionless, the fore legs extended and the hind legs 

 pendent, though very slightly flexed. If such a frog were dissected down to 

 the nervous system, there would be found the following arrangement : 

 Afferent fibres running from the skin, muscles, and tendons, and entering the 

 cord by way of the dorsal nerve-roots. The central mass of the spinal cord 

 itself in which these roots end, each root marking the middle of a segment. 

 Within the cord and stretching its entire length are to be found the central 

 cells, interpolated more or less numerously between the terminals of the 

 afferent neurones and the cell-bodies of the efferent neurones. From each 

 segment of the cord go the ventral root-fibres passing in part to the muscles 

 and in part to the ganglia of the sympathetic system. The mechanism 

 demanded for a reflex response is an afferent path leading to the cord ; cells 

 in the cord by which the incoming impulses shall be there distributed; and 

 a third set of efferent elements to carry the outgoing impulses to the 1 terminal 

 organ which gives the response. It is important to consider in detail what 

 occurs in each portion of this reflex arc. 



In a frog thus prepared, stimulation of the skin in any part supplied by 

 the sensory nerves originating from the spinal cord causes a contraction of 

 some muscles. 



Influence of Location of Stimulus. — The muscles which thus contract 

 tend to be those innervated from the same segments of the cord which receive 

 the sensory nerves that have been stimulated. Thus stimulation of the skin 

 of the breast causes movements of the fore limbs, and stimulation of the rump 

 or legs corresponding movements of the hind limbs. It is noticeable, how- 

 ever, that wherever the stimulus i> applied the hind Limbs have a tendency 

 to move at tlie same time that the muscles most directly concerned contract, 

 [f the attempt is made to correlate these variations in reaction with varia- 

 tions in the structure of the cord, we have to picture the simplest reactions 

 (from the same level) as dependent on the formation of terminals on the 

 afferent fibre just after its entrance into the cord and in the immediate neigh- 

 borhood of an efferent neurone. In the second case either the afferent axone 

 i- extended some distance through the cord forming several terminations by 

 Vol. ir. -u 



