92 THE CENTRAL NERVOUS SYSTEM 



extended, and their nature almost infinitely varied, by the needs and 

 training of special trades and professions. It will be sufficient for our 

 purpose to sketch in a few words the mechanism of one or two of the 

 most common and fundamental co-ordinations of muscular effort, 

 passing over the rest with the general statement that the more refined 

 and complex movements are in general brought about, not by the abrupt 

 contraction of crude anatomical groups of muscles, but by the contrac- 

 tion of portions of muscles, perhaps even single fibres or small bundles 

 of fibres, while the rest remain relaxed. The excitation may gradually 

 wax and wane as the different stages of the movement require. Antago- 

 nistic muscles may be called into play to balance and tone down a con- 

 traction which might otherwise be too abrupt. 



Many interesting illustrations of this process of ' give and take ' 

 between opposing muscles have been reported, especially by Sherring- 

 ton. Some have been already alluded to in discussing reflex move- 

 ments (p. 903). One or two additional observations may be given here. 

 In the cortex cerebri, as we shall see (pp. 951, 965), there is an area in 

 the frontal region, and another in the occipital region, stimulation of 

 which gives rise to conjugate deviation of the eyes that is, rotation of 

 both eyes to the opposite side. Sherrington divided the third and 

 fourth cranial nerves in monkeys say on the left side. The external 

 rectus, which is supplied by the sixth nerve, caused now by its unopposed 

 contraction external squint of the left eye. When either of the cortical 

 areas referred to, or even the subjacent portion of the corona radiata, 

 was stimulated on the left side, both eyes moved towards the right, the 

 left eye, however, only reaching the middle line that is, the position in 

 which it looked straight forward. The same thing was observed when 

 the animal, after complete recovery from the operation, was caused to 

 voluntarily turn its eyes to the right by the sight of food. Here an 

 inhibitory influence must have descended the fibres of the abducens, the 

 only nervous path connected with the extrinsic muscles of the left eye, 

 and the relaxation of the left external rectus must have kept accurate 

 step with the contraction of the right internal rectus. Hering has made 

 an exhaustive analysis of the co-ordinated movements concerned in 

 opening and closing the hand in monkeys. These movements can be 

 produced by stimulation of the cortex or the internal capsule, but not 

 by stimulation of the anterior spinal roots. When the hand is opened 

 the muscles that open it are excited, and those which close it are in- 

 hibited from the cortex. 



Reaction Time. Just as in a reflex act a certain measureable 

 time (reflex time) is taken up by the changes that occur in the lower 

 nervous centres, so we may assume that in all psychical processes 

 the element of time is involved. And, indeed, when the interval 

 that elapses between the application of a stimulus and the signal 

 which announces that it has been felt (reaction time) is measured, 

 it is found that for the cerebral processes associated with the per- 

 ception of the simplest sensation and the production of the simplest 

 voluntary contraction it is longer than the time which the spinal 

 centres require for the elaboration of even complex and co-ordinated 

 reflex movements. Suppose, e.g., that the stimulus is an induction 

 shock applied to a given point of the skin, and that the signal is the 

 closing of the circuit of an electro-magnet, then, if both events are 



