BEOWS-SEQTJARD ON THE CEXTEAL NEETOTJS SYSTEM. 279 



pricking with this small point will be more delicately appreciated in 

 some parts than in others, and if the front of the forearm be irritated 

 by the application of a sinapism for a few minutes, we can easily 

 satisfy ourselves, by comparison with the other forearm, that, so far 

 as regards pricking, the sinapism has produced hyperesthesia. These 

 obvious facts place within our reach means, at once ready and toler- 

 ably precise, for making comparisons between symmetrical and other 

 parts of the surface ; but for measuring the degree of tactile sensi- 

 bility, perhaps the best method of all is that indicated by Dr. Brown- 

 Sequard in his " Experimental Eesearches." The curious facts 

 discovered by E. H. Weber, respecting tactile sensibility are well 

 known. He found that if the blunted points of a pair of compasses 

 are applied simultaneously on the skin, there is, according to circum- 

 stances, either the sensation of one or of two points. When the points 

 are both inside of certain boundaries, they are felt as one only, when 

 they are outside of these boundaries, both are felt. These boundaries 

 vary exceedingly in different parts of the skin, but, for a given part, 

 the differences between various individuals are not considerable. The 

 compasses may be made use of for measuring the degree of tactile 

 sensibility in diseases : in a case of considerable anesthesia of the 

 lower limbs, the patient only felt a single impression on one leg, 

 although the points of the compass were ten, fifteen, or even twenty 

 centimetres apart, whilst, on the other leg, he could distinguish 

 them at a distance of twelve centimetres. The normal limit for 

 that limb is generally from three to five centimetres. In another 

 case where anaesthesia was slighter, the limit of the discriminating 

 power was at from nine to sixteen, centimetres. In two other cases, 

 in which the diminution of sensibility had not been discovered by other 

 means of diagnosis, the compass indicated a very slight and beginning 

 anaesthesia ; the limit being at from six to seven centimetres. 



These facts demonstrate that, by the help of a pair of compasses, 

 a physician can with tolerable precision determine : 1st, whether there 

 may be slight anaesthesia or not. 2nd, what is the degree of anaes- 

 thesia. 3rd, what changes occur from day to day in the amount of 

 anaesthesia as regards tactile sensibility. 



The same is true for cases of hyperaesthesia. In a case of para- 

 plegia jof motion, the patient felt the two points of the compasses on 

 his feet, even at the distance of five millimetres, whilst a healthy 

 person feels the two points only when they are at a greater distance 

 than twenty-five millimetres. For success in such experiments the 

 two points should be blunted and applied simultaneously. 



To how great a degree this tactile sensibility is capable of im- 

 provement by education, is known to every one who has visited insti- 

 tutions for the reception of blind persons ; yet, in making compara- 

 tive experiments on different individuals, or even different parts of 

 the same person, it should not be forgotten how much its delicacy 

 may depend upon the condition of the skin itself, quite independently 

 of the nervous system. 



TOL. I. — N. H. K. 2 O 



