946 THE CENTRAL NERVOUS SYSTEM 



a great extent of a motor machinery already existing in the cord and 

 capable of discharging well co-ordinated reflexes. In addition to such 

 fundamental movements, most people consciously learn, and are 

 willing to confess that they have learnt, to execute a considerable 

 number of co-ordinated movements with the arms, and especially with 

 the fingers. Some part even of the extreme dexterity of jaws, tongue, 

 and teeth displayed by a hungry school-boy, in a minor degree, perhaps, 

 by a hungry mouse, is the result of the much practice, entailing at first 

 some conscious effort, which maketh perfect. The exquisite co-ordina- 

 tion of the muscles of the eyeball, which we shall afterwards have to 

 speak of, and the no less wonderful balance of effort and resistance, of 

 power put forth and work to be done, of which we have already had 

 glimpses in studying the mechanism of voice and speech, become to a 

 great extent the common property of all fully-developed persons. But 

 the technique of the finished singer or musician, of the swordsman or 

 acrobat, and even the operative skill of the surgeon, are in large part the 

 outcome of a special and acquired agility of mind or body, in virtue of 

 which highly-complicated co-ordinated movements are promptly deter- 

 mined on and immediately executed. 



With such special and elaborate movements it is impossible to occupy 

 ourselves in a book like this. Their number may be almost indefinitely 

 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. 875). One or two additional observations may be given here. 

 In the cortex cerebri, as we shall see (pp. 919, 931), 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 

 rect us, 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. Hcring 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 



