52 CONNECTIVE TISSUES. 



Fixed Corpuscles of the Cornea. In a cornea prepared 

 in the manner previously described, it is possible to recognize 

 the network of pale branched corpuscles at all depths, after 

 some time has elapsed. They may, however, be more distinctly 

 shown with the aid of certain reagents, particularly wood 

 vinegar, nitrate of silver, chloride of gold, and some other 

 metallic salts. Of these, the first is now laid aside in favor of 

 the others. In preparations obtained by stripping off shreds of 

 a cornea (of the rabbit or frog), which has been macerated for 

 twenty-four hours in wood vinegar, the corpuscles are seen as, 

 large flattened cells, consisting of graaiulo.us protoplasm, com- 

 municating with one another by processes. If vertical sections 

 are made of such a cornea, the cells seem to be spindle-shaped ; 

 but, if the section is made obliquely, it is found that the cor- 

 puscles appear the more flattened and the more branched, the 

 greater the obliquity of the section. This fact proves that the 

 corpuscles are flattened in planes parallel to the surface, and 

 that the processes also stretch out in similar planes. 



Treatment of the Cornea with Nitrate of Silver. 

 Nitrate of silver is used both in substance and in solution. In 

 substance it may be employed in two ways : a. The centre of 

 the cornea of a frog, which is held by an assistant in the 

 manner previously described, is firmly cauterized with a pointed 

 stick of lunar caustic. One or two drops of salt solution are 

 then allowed to flow over the cornea to decompose the excess 

 of nitrate of silver. About an hour after the cauterization, 

 the cornea is excised in the manner directed in p. 49, washed 

 in water for several minutes, and the surface of the slough 

 cleansed by pencilling it lightly under water. In the case of 

 the frog's cornea, the central cauterized part may be cut out 

 and mounted in glycerin at once; but the rabbit's cornea is so 

 thick that it is necessary to split it into layers, with the help 

 of fine pointed forceps. If the preparation has been exposed 

 to daylight, clear spaces are seen on a brown, yellow, or dark 

 ground, which communicate with one another by clear channels, 

 either branched or single. These correspond in form and 

 configuration with the network of corpuscles above described. 

 This signifies that we have before us, as will be more com- 

 pletely shown afterwards, the spaces which the corpuscles 

 occupy. This network of clear spaces represents the canalicular 

 system (Saftcandlchen System) of the cornea: it must not be 

 confused with Bowman's tubes, b. The second method of ap- 

 plying the nitrate of silver in substance has the advantage 

 that it shows the canalicular system in all parts of the cornea. 

 It consists in first scraping the cornea of a living frog or small 

 mammal with a sharp cataract knife, so as to remove the epi- 

 thelium completely. After a little practise, and provided ther 

 bulb is properly fixed by an assistant, it is not difficult to per- 



