838 A MANUAL OF PHYSIOLOGY 



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 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. 



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 80 1) . One or two additional observations may be given here. 

 In the cortex cerebri, as we shall see (pp. 845, 859), 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, w r as 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 con- 

 cerned in opening and closing the hand in monkeys. These move- 

 ments 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 inhibited from the cortex. 



Standing. In the upright posture the body is supported chiefly 

 by non-muscular structures, the bones and ligaments. But muscles 

 also play an essential part, for it is only peculiarly-gifted individuals, 

 like some of the fishermen of the North Sea, who can go to sleep on 

 their feet, and a dead body cannot be made to stand erect. The 

 condition of equilibrium is that the perpendicular dropped from the 

 centre of gravity to the ground should fall within the base of support 



