124 



whenever the least symptoms of a toxic influence made their appear- 

 ance, a ligature placed over the limb rarely succeeded in saving the 

 animal. 



In order to obtain results more susceptible of measurement, I pro- 

 ceeded to substitute atropia for aconite, and to make use of the albino 

 rat in lieu of the guinea pig. By this means, I possessed an agent 

 whose intervention was immediately detected by its action on the 

 iris. My choice of the albino rat was for the like reason, f. e. the 

 facility which it offered for exact and easy measurement, in which 

 respect this animal is far preferable to any other with which I am 

 acquainted, unless we except the white mouse, which, however, is so 

 liable to die from slight causes, that it is little adapted for most 

 physiological experiments. 



The modus operandi which I generally adopt is to immerse the 

 limb into a small 2-drachm bottle containing sufficient of the mixture 

 to cover the foot and part of the leg. The strength of the solution 

 of atropia being generally that from half a grain to one drachm of 

 some menstruum, such as chloroform, alcohol, &c., I generally prefer 

 simply to hold the animal during the experiment to any other mode 

 of restraint. By these means I am able to guard against several 

 causes of error, such as the direct contact of the solution with the 

 eye or mouth, and, at the same time, avoid any unnecessary dis- 

 comfort to the animal. 



Chloroform and Atropia. A solution of atropia in chloroform 

 will generally be found to cause dilatation of the pupil after the foot 

 has been immersed from two to five minutes. The dilatation, having 

 once commenced, is usually very rapid, and the pupil very soon attains 

 double or treble its normal diameter, which is about J to ^ a millimetre 

 during day-time. It is easy to recognize that this dilatation is not in 

 very simple ratio to the time occupied in its expansion, the expansion 

 of the pupil being more nearly in proportion to the square of the time 

 occupied than in a simple arithmetical ratio. Immersion of one limb 

 causes both pupils to dilate equally, except in some few instances, 

 where one pupil expands much more than the other, from some con- 

 stitutional peculiarity, which remains the same whichever foot be 

 immersed. 



Although I have never failed to obtain dilatation of the pupils by 

 the immersion of the foot in this solution of atropia, yet, in some 



