REFLEX ACTION. 



301 



The cord is to be regarded as a longitudinal series of simple automatic 

 nerve arcs, or, as we have termed it, a multiple automatic Automatic ac- 

 arc. Each segment of it has therefore an independent action tionofthecord. 

 of its own, but can conspire with its neighbors or be influenced by the 



brain, by means of its commissural 

 fibres, an arrangement of which num- 

 berless interesting instances might be 

 furnished. The one represented in 

 Fig. 144, which is from the cord of 

 spirostreptus, may, however, suffice: 

 A, under surface of a portion ; B, up- 

 per surface; a, inferior longitudinal 

 fibres ; e, superior longitudinal fibres ; 

 y, fibres of re-enforcement, seen also 

 at b and c\ g, commissural fibres, seen 

 also at d. 



The power which the cord displays 

 in this simple action is most striking- 

 ly seen when it is cut off from its 

 cranial connections. The decapitated 

 frog props himself up stiffly on his 

 legs, and, if his cutaneous surface be 

 irritated, exhibits antagonizing mo- 

 tions ; such motions are all of the reflex character, and are commonly 

 much more strikingly seen in cold than in warm-blooded animals ; but 

 even in man precisely the same results are witnessed during periods of 

 the suspension of the activity of the brain, as, when the palm of the hand 

 of a sleeping child is touched with the finger, the finger is at once grasped. 

 As above stated, this reflex function of the cord is therefore independ- 

 ent of the brain, though the brain can control it, and this Reflex action 

 the more perfectly the higher the organization of the animal, independent of 

 Breathing can go on, whether we pay attention to it or not, * 

 but we can arrest it if we choose for a time ; and since in man this in- 

 troduction of air is incidentally used for very refined purposes, by volun- 

 tary exertion we moderate or regulate it, as in the production of musical 

 sounds in singing or of articulate sounds in speech. 



In a general way, there is not much difficulty in distinguishing be- 

 tween simple actions of the cord and those in which the brain Distinction be- 

 is participating. In the former, no weariness or fatigue is ^cerebral 

 ever experienced ; in the latter it is ; and perhaps, even in action, 

 these last, involving voluntary muscular action, though the control is to 

 be attributed to the brain, the source of the force is in the cord. 



These normal phenomena which the cord displays become greatly ex- 



Portion of cord of spirostreptus. 



