716 



FUNCTIONS OF THE SPINAL COED 



[CH. XL1X. 



experiment in which the posterior roots are divided, and although 

 some fibres may remain which still allow of the passage of nervous 

 impulses, the action of the three circles is greatly interfered with ; 

 the spinal reflex arc is at fault ; this is shown by the loss of reflex 

 action, the disappearance of the tendon reflexes, and the want of 

 tonus in antagonistic muscles ; the main symptom of the disease is 

 want of muscular coordination, and this is produced not only by the 

 lesion in the spinal cord, but is accentuated by the want of continuity 

 in the other two circles, so that the brain is unable to effectively 

 control the motor discharge from the anterior cornual cells. 



M'DougalVs "Drainage" theory. This theory is an attempt to explain the 

 reciprocal action of antagonistic muscles. 



The accompanying diagram represents two antagonistic muscles (fig. 442) with 



FLEXOR 



EXTENSOR 



Fio. 442. Diagram to illustrate M'Dougall's "Drainage" theory. 



their nerve supplies. Each is in connection with a reflex arc shown in a simple 

 schematic way, as consisting of three neurons, A 15 A 2 , A 3 , and B lf B 2 , and B 3 

 respectively. 



A! and B x are the afferent neurons ; 



A 2 and B 2 are the association or internuncial neurons within the central nervous 

 system. 



A 3 and B 3 are the efferent or motor neurons. 



When a stimulus is applied to A l it generates nervous energy, and discharges 

 across the synapse to A 2 , and finally to A 3 , and the muscle contracts. The problem 

 then is to imagine such a mode of connection between arc A and arc B as will 

 cause arc A during activity to drain off from arc B the smaller amount of nerve 

 energy in it which normally keeps the muscle supplied by B 3 in a state of tonus ; 

 if this is done, the muscle of arc B will lose its tonicity and become relaxed. 

 It is probable that this connection is by means of a collateral of the internuncial 

 neuron B 2 crossing, as shown in the figure and taking part with the axon of A 2 in 

 the formation of the synapse with A 3 . The normal resistance of this synapse is 

 lowered by the stimulation applied to the arc A, and this lowering of resistance is 

 participated in by the part of the synapse to which the crossing collateral from B 2 

 contributes ; owing to this lowering of resistance, nervous energy therefore drains 

 over from the arc B, and so the muscle it supplies is relaxed. 



The theory may also be applied to explain (1) alternating reflexes, as described 

 under our description of the scratch reflex (p. 712) ; (2) the reinforcement of the 

 knee-jerk and other phenomena in which reflex arcs are concerned (p. 708) ; and 

 (3) certain psychological phenomena such as attention. 



The Principle of the Common Path (Sherrington). When an 

 afferent nerve is stimulated, the impulse enters that complex network 



