50 CONNECTIVE TISSUES. 



trated with a cataract knife, just as in the operation for cata- 

 ract, at the limbus conjunctivas next the inner canthus. The 

 point of the knife is advanced until it approaches the limbus 

 of the opposite side, without puncturing it, and is then carried 

 outwards and upwards, so as to form a flap, consisting of the 

 upper half of the cornea. The extreme edge of the flap must 

 then be seized with the forceps, while the lower half of the 

 cornea is cut awa} r with the aid of scissors curved in the direc- 

 tion of their edge. The cornea is next transferred to a drop 

 of humor aqueus (previously obtained by puncturing the oppo- 

 site eye) and spread out on the glass slide with the anterior 

 surface uppermost. In order to avoid folds, it is desirable 

 to make two or three radical incisions. The preparation is now 

 covered and inclosed in oil. If it is desired to study the mi- 

 gratory cells on the warm stage, the preparation must of 

 course be mounted between two cover-glasses, as before di- 

 rected. 



If a cornea is thus prepared with great care, nothing is to 

 be seen excepting that a few pale lines of interstitial substance, 

 referable to the anterior epithelium, may be distinguished where 

 the membrane is folded. No other optical differences can be 

 made out. If the individual epithelial elements can be distin- 

 guished, this affords proof that the object has been injured in 

 preparation. Notwithstanding its homogeneity, it is possible 

 (with the No. 10 immersion objective of Hartnack) to find out 

 the upper and under surfaces of the cornea by means of colored 

 blood corpuscles, pigment, granules, or retina-elements which 

 may happen to be in contact with them. As time goes on, the 

 interstitial lines of the anterior epithelium come into view. If 

 we then adjust the microscope so as to bring into view the 

 most superficial layer of the propria, a few corpuscles of more 

 or less irregular form can be detected, each of which consists 

 of almost hyaline protoplasm, and contains a nucleus of ir- 

 regular form, apparently finely granular. If one of these cor- 

 puscles is watched carefullj', it is seen that changes of form 

 take place, both in the protoplasm and in the nucleus. The 

 corpuscles throw out processes and retract them, and even per- 

 form a certain amount of locomotion. The nuclei become con- 

 stricted or compressed, and again resume their original form, 

 to undergo similar changes. By and by similar corpuscles 

 become visible in the depth of the cornea. On the warm stage 

 the movements are naturally more active. (See Chapter 

 XIY.) 



If a preparation is made in humor aqueus of the fresh peri- 

 toneum (particularly the omentum) of the frog or of a mammal, 

 or of a septum of a subcutaneous lymph sac of the former, in- 

 numerable migratory cells are seen, especially in the neighbor- 

 hood of the vessels, which present transitions between small 



