258 PRACTICAL HISTOLOGY. 



purpose. A solution of alkanet in turpentine is the 

 fluid which should be chosen, and the mercurial 

 pressure apparatus (Fig. 30) is used. The tube and 

 fine steel canula having been filled with the alkanet 

 solution to the exclusion of air-bubbles, the canula is 

 inserted obliquely into the substance of the cornea, 

 without allowing the point to pass through into the 

 anterior chamber. The pressure is then gradually 

 raised to about two inches of mercury, when the red 

 fluid should begin gradually to fill the cell-spaces, 

 and to spread through them over a considerable part 

 of the cornea. Indeed, if the injection is long enough 

 continued, the fluid may extend itself even beyond 

 the cornea 1 margin, and may penetrate into the cell- 

 spaces in the anterior part of the sclerotic coat. The 

 operation may with care be successfully performed 

 without the mercurial apparatus, using merely a 

 Pravaz syringe. But it is difficult to avoid the pro- 

 duction of extravasation near the point of the canula. 

 This does not, however, always militate against the 

 success of the experiment, for beyond the limit of 

 the extravasation the fluid may slowly penetrate 

 into the cell-spaces of the tissue, and this may go 

 on even after the syringe has been removed, espe- 

 cially if the cornea is cut out, laid flat on a slide, 

 and allowed to dry. As watery fluid becomes with- 

 drawn from the cell-spaces in the process of drying, 

 the alkanet solution tends to pass in to occupy its 

 place. 



Another very successful plan of inducing the in- 

 jecting fluid to pass from such an extravasation into 

 the neighboring cell-spaces consists in gently stroking 

 from the extravasation towards the margin of the 

 cornea with a smooth instrument, such as a glass 

 rod or the ivory handle of a scalpel. But if too 

 great pressure is exerted upon the fluid it will be 

 found that, in place of taking the closely reticulating 

 course which it would pursue if it merely occupied 

 the cell-spaces, the injection tends to shoot through 

 the tissue in straight lines, which in successive planes 



