190 FUNCTIONS OF THE NERVOUS SYSTEM. 



object, there is an instinctive tendency, on the part of the Nervous System 

 and the Muscles by which it acts, to effect a consensual movement of the other ; 

 so that its accustomed axis also shall be directed towards the object. This 

 principle fully accounts for the only non-consensual movement which can be 

 performed in any way voluntarily, that of both eyes inwards, or downwards 

 and inwards, which is effected by the conjoint action of the Internal Recti. 

 Some persons possess the power of performing this to a much greater degree 

 than others ; but in all instances, the eyes must be fixed on an object; and 

 thus the movement is as referable as any other to this principle. It is, per- 

 haps, desirable to qualify the classification of the nerves and muscles of the 

 Orbit proposed by Valentin, by admitting that all have, in some degree, a 

 voluntary, and in some degree an automatic action ; but that voluntary power 

 predominates in regard to one group, whilst the other is more commonly acted 

 on by an automatic impulse. It is clear that the will must have some power 

 over the Inferior Rectus, for example ; since both eyes can be voluntarily 

 directed downwards. But the power of the Will over this muscle is much 

 less than it is over the Superior Rectus ; as is shown by the fact that, if we 

 direct the eyes downwards, and then close the lids, no effort of volition can 

 prevent the eye from being rolled upwards by the Inferior Oblique ; and that, 

 whilst the lids remain closed, the pupils cannot be directed downwards in any 

 considerable degree. It is evident, then, that the impression of an object upon 

 the retina is almost as necessary to occasion the combined action of the Infe- 

 rior as it is to produce that of the Internal Recti. The case is very different 

 in regard to the Superior Recti, which can be made to act together, in any 

 degree, without the necessity of a visual impression. 



256. It has been pointed out by Mliller, as an obvious reason for the sepa- 

 ration of the 6th from the 3d pair of nerves, that there is usually a great 

 tendency to consentient action between the nerves of the two sides, which 

 pass off from the same point of the cerebro-spinal axis as we see in the case 

 of Reflex movements of both sides (such as that of the pupil), which are 

 excited by a stimulus applied to one only ; and that this holds good also in 

 those movements of the eyes which are effected by the third pair exclusively 

 such as the elevation or depression of both pupils ; but in the horizontal 

 movements of the eyeballs, two different actions are being performed on the 

 two sides respectively; and it may be conceived that this may be more readily 

 accomplished by two different nerves than by branches of the same. We may 

 admit some truth in this idea, without attributing much weight to it. It has 

 been already stated as a result of Embryological research, that all the Nerves 

 of the Orbit do in reality form part of the Spinal nerve, to which the Fifth 

 pair alone has been commonly regarded as equivalent ; and it is well known 

 that we can perform many different actions on the two sides, through the 

 medium of similar nerves at the same time. It is remarkable, however, that 

 there are some dissimilar movements which it is impossible to execute with 

 any degree of rapidity, except by long practice : thus, if we move the right hand 

 as if winding on a reel, and afterwards make the left hand revolve in a con- 

 trary direction, no difficulty is experienced; but if we attempt to move the two 

 at the same time, in contrary directions, we shall find it almost impracticable. 



257. There can be no doubt that, in these and many other voluntary move- 

 ments, we are guided by the sensations communicated through the afferent 

 nerves, which indicate to the mind the state of the muscle. Many interesting 

 cases are on record, which show the necessity of this muscular sense, for 

 determining voluntary contraction of the muscle. Thus Sir C. Bell (who pro- 

 minently directed attention to this class of facts, under the designation of the 

 Nervous Circle,) mentions an instance of a woman who was deprived of it in 

 her arms, without losing the motor power, and who stated that she was 



