442 



Messrs. L. Hill and M. Flack. 



[May 29, 



pressure and lessens the tension of the eyeball considerably. Thomson 

 Henderson recognised the need for the invention of a proper needle, but 

 opportunity did Dot allow him to continue this research. The L.H. needle is 

 cone-shaped and somewhat larger than a hypodermic needle ; the cornea grips 

 it tightly, and since it is closed no fluid escapes at the moment of insertion. 

 With practice, we have come to introduce the needle, as a rule, without leak. 

 In our earliest experiments we often failed to do so and obtained readings 

 which were too low. A possible source of error in the method arises from the 

 volume of the needle introduced within the eyeball. If blood and not aqueous 

 fluid is displaced from the eyeball in an amount corresponding to the bulk of 

 the needle introduced, the intraocular pressure may rise and the readings be 

 too high. The needle might act as does a clot, or a foreign body, when 

 introduced into the intracranial cavity ; by taking up room a clot expresses 

 blood from the cerebral veins and increases the pressure in the exsanguined 

 part from capillary-venous towards arterial pressure. If the bulk be small 

 compared to the whole the result will be negligible. We have tested whether 

 this be so by comparing the readings taken with the L.H. needle inserted 

 in one eye and a hypodermic needle inserted in the other, the latter reading 

 being taken immediately after insertion of the needle, before leakage has taken 

 place. When introduced with a sudden thrust we obtain, with a small 

 hypodermic needle, readings quite as high as with the L.H. needle. In some 

 cases the two needles were inserted into the same eye. Since these readings 

 (Table I) closely corresponded we have no reason to doubt the accuracy of 

 our method. 



Table I. 



Animal. 



Anaesthetic. 



L.H. needle. 



Hypodermic 

 needle. 



Eemarks. 



Cat 



Ether 



mm. Hg. 

 54 



| 53 

 51 



50 



36 

 16 

 15 ' 



mm. Hg. 



52 



46 | 

 49-8 



50 -5 



36 

 15 

 15 



B.P. 100. Eeadings taken in 

 different eyes. Hypodermic 

 quickly fell to IS mm. Hg. 



Both readings in same eye. 

 When hypodermic read 46, 

 L.H. also read 46 mm. Hg. 



Needles in opposite eyes. 

 Hypodermic gradually sank 

 to 30 mm. Hg. 



Needles in opposite eyes. 



Eight eye. 1 Needles in same 

 Left „ J eye. 



Dog 



t Ether 



Cat 



Ether 



Cat 



Ether 



Kitten 





Dog 



Dead 







The average intraocular pressure as measured by 15 or more observers 

 is 20-30 mm. Hg. In several of our animals, especially those anaesthetised 



