SENSE OF VISION. 881 



vical portion of the sympathetic might afford the dilating power. By galvaniz- 

 ing the third pair and the cervical sympathetic alternately, alternate contraction 

 and dilatation of the pupil are produced ; but with this difference, that the contrac- 

 tion is immediate, as is also the return of the iris to its previous state after the 

 irritation is withdrawn, whilst, on the other hand, the dilatation requires some 

 seconds to attain its maximum, and the subsequent return of the iris to its pre- 

 vious condition is slow. Moreover, the third pair loses its power of conducting 

 irritation after being several times excited in this manner, and also very soon 

 after death ; whilst the sympathetic retains it longer. 



The authors next applied themselves to determine the precise centre of this 

 power, and they ascertained, in the first place, by galvanizing the undivided 

 trunk in the rabbit, that irritation of any part of the cervical portion of the 

 sympathetic, from the first to the last ganglion inclusive, would produce this 

 result; but that below the last cervical ganglion, no such effect could be pro- 

 duced by irritation. On exposing the spinal cord, they found that irritation of 

 any part of it between the first cervical and sixth dorsal vertebrae (which they 

 hence designate the cilio-spinal region) would produce dilatation of the pupil ; 

 but that the effect was most decided in the centre of this region, the maximum 

 being at the junction of the second and third dorsal vertebrae, and decreasing in 

 intensity on passing towards either of its limits. When both of the sympathetic 

 trunks are entire, irritation of this region produces contraction in both pupils 

 equally; but when one of the sympathetic trunks is divided, the dilatation does 

 not take place on that side ; and if both be divided, no irritation of the spinal 

 cord produces any effect on the pupil. It is sufficient to irritate one side of the 

 spinal cord only, to produce dilatation of both pupils ; but if the spinal cord be 

 divided longitudinally, and the two halves be kept asunder by a slip of glass, 

 irritation of one side then acts only on the pupil of that side. When the spinal 

 cord is divided transversely at different points, it is found that irritation of any 

 part that is isolated from the centre of the cilio-spinal region is unproductive of 

 effect. When the spinal cord was removed, and the galvanism was applied to 

 the dura mater lining its canal, this limitation was still more precise; for it was 

 then found that in no other point was any effect produced than at the inter- 

 vertebral space between the second and third dorsal vertebrae. Thus it seems 

 obvious that the motor fibres on which this action depends issue from the spinal 

 cord with the second dorsal nerve ; and this corresponds with the results of 

 dissection, as compared with the lowest limit of the influence of the sympa- 

 thetic trunk over the movement of the pupil. All the causes which diminish 

 muscular irritability after death, such as imperfect nutrition of the animal 

 lesions, of the medulla oblongata, &c., dimmish or destroy the power of the 

 cilio-spinal region over the pupils; and in all cases it is to be noted that after 

 death the power gradually recedes from the extremities of the cilio-spinal region 

 towards its centre. 



The authors then proceed to study the action of the intercranial nerves ; and, 

 in the first place, that of the fifth pair, respecting which there has been so much 

 contrariety of opinion. In order to remove all sources of fallacy so far as possi- 

 ble, they have divided all the other nerves connected with the eye (including 

 the cervical sympathetic); and they have then found that sections of the oph- 

 thalmic branch of the fifth pair, or of the trunk from which it is given off, 

 causes a slow and gradual constriction, which does not commence for a minute 

 or two after the section, and does not attain its maximum for as much more. 

 The constriction is very decided, the pupil being reduced from three lines in 

 diameter to one. If, instead of division of the nerve, mechanical or galvanic 

 irritation be applied to it, the same effect is produced, but in a less degree. 

 The constriction is in no way permanent, but departs gradually. After section 

 of the optic nerve, section of the third pair does not produce any change in the 

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