736 A MANUAL OF PHYSIOLOGY 



such as walking and the co-ordinated muscular contractions involved 

 in standing, and even in sitting, which, once acquired, appear so 

 natural and spontaneous, have to be learnt by painful effort in the 

 hard school of (infantile) experience. Most people 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 ; but few have considered that the extreme dexterity 

 of jaws, tongue, and teeth displayed by a hungry mouse or school- 

 boy is the result of the much practice which maketh perfect. The 

 exquisite co-ordination 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 determined 

 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 contraction 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. Antagonistic muscles may be called into 

 play to balance and tone down a contraction which might otherwise 

 be too abrupt. 



A most interesting illustration of this process of * give and take ' 

 between opposing muscles has been reported by Sherrington. In 

 the cortex cerebri, as we shall see (pp. 743, 747), 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 re- 

 covery from the operation, was caused to voluntarily turn its eyes to 



